The 18650 will have more capacity and lower internal resistance.
You just need to replace the springs or bypass them to get the full potential out of the C8. Boost drivers request can pull even higher currents than a FET driver by upping the voltage.
Lowering the resistance at any point in the flashlight itself is crucial.
My thoughts: a quality 18650 would have so much less resistance than 2 18350’s that a FET wouldn’t be pulling nearly as much as a high boost driver. I guess it all depends on what the amps are set at on the boost driver, right?
Yes, but I need to explain myself a bit.
In a single cell light, a boost driver will pull more current as the voltage drops farther and farther from the emitter’s foward voltage to keep it in regulation.
That means using steel springs not bypassed at say 6A current, the voltage drop will be around 0,3V. Which is huge.
Say the output power is 18W for a 6V emitter
Our cell at varying voltages and current draw:
18W= 4,20V x 4,28A
18W = 3,9V x 4,50A
18W = 3,60V x 5,00A
As you can see, current draw increases as voltage decreases.
Now, let’s introduce a steel spring(–0,3V for every voltage)
18W = 3,9V x 4,62A
18W = 3,6V x 5,00A
18W = 3,3V x 5,45A
That means you have to pull even more current out of the cell.
Imagine that but at 50W power! You would be pulling at least 12A at 4,2V, and that could go up to 14A at 3,6V, or even 15A at 3,3V! With only a steel spring and no bypass, that would be go much worse. And that is not even counting the internal resistance of the cell, and other resistance in the circuit.
You’d probably get either a low voltage warning almost instantly due to the huge spring resistance, a burned spring, or a huge power loss.
That problem is still there with a FET driver, but since current pulled goes down with voltage, it isn’t as much trouble.
TLDR: For such a powerful boost driver, going with a single 18650 is crucial for both energy and power density. You also need to get rid of all almost all contact resistance, which includes the springs.
Edit: Remove something I should not have said :person_facepalming:
We get it we all bought your springs Mr BlueSpringM 
I have a BLF D80 that has been waiting for a decent driver for a XHP50.2 build. can’t wait!
How have you not burned out the 50.2 yet? Those leds don’t like FET drivers. Are you running very low drain 18350’s?
FM
They are also useful for running triples and quads in series configs. Particularly if one wants to mix emitters with different forward voltages.
I got the clear anodizing C8… its a fun and good looking light!
BlueSword - thanks for the detailed explanation! I may have to get in on your spring group buy! 
Well I guess I jinxed myself. I was using the above C8 tonight, since we were talking about it… all of a sudden it decided it only wants to use moonlight mode for all levels of intensity. Low is like 1 lumen, medium 10 lumens, high 50 lumens.
Looking at the led on low, one of the arrays is brighter than the other 3. Did I finally blow the emitter?
So I just checked mountain’s website and I see an out of stock listing for a 17mm boost driver!
What, no link? Lol
This boost driver (input voltage is lower than output voltage) can drive 6V LEDs, such as the XHP50.2/70.2 from a single rechargeable lithium-ion battery at an output current of up to 4.2A. It can also drive a 12V LED, such as the XHP35, at up to around 1.3A. The MTN-BST2 uses a highly efficient synchronous boost converter squeezed into a tiny 17mm form factor.
Unlike most of the high power boost drivers out there, this driver can achieve a wide dimming range and utilize popular firmwares.
This driver takes up to 10 business days to be built, tested, and shipped. Your order will not ship until the driver is completed.
Battery & Host Requirements:
In stock form, this driver will pull just over 10A peak current, therefore a 10A+ continuous rated battery is required (unless you reduce the input current limit). The more voltage the battery can hold under load, the better the driver will perform. Your switch, springs, etc. must be able to handle the 10A+ input current; this means that you must have high performance or bypassed springs in your light.
The driver _must_ be soldered into the pill or held firmly in place with a retaining ring. This is required both for adequate thermal and electrical performance. If during use the light turns off and on, the driver is overheating and needs to be switched to a lower mode or turned off and allowed to cool. Continued operation at excessively high temperature will damage the driver (damage from excessive temperature will not be covered under warranty).
Specifications:
Boost Converter (input voltage must be lower than output voltage)
3V - 5.5V input voltage range (suitable for a single lithium-ion battery)
Designed to drive 6V LEDs, such as the XHP50.2 and XHP70.2 at drive currents up to 4.5A
Can also drive 12V LEDs, such as the XHP35 at currents up to around 1.3A
Output voltage range: ~6V —- ~13.8V
Output current range: 6V LED: 1A —- 4.2A
XHP50 / 50.2
XHP70 / 70.2
MT-G2 6V
12V LED: ~1.3A
XHP35 HD / HI
Input Current Limit Option:
If you are running any battery that is not capable of 10A+ continuous discharge current, you must change the input current limit to a lower value.
~10.5A | 4.2A w/ 6V LED maximum recommended output current
~8A | 3.5A w/ 6V LED maximum recommended output currnet
~4A | 2A w/ 6V LED maximum recommended output current
Output Current Option:
The output current is set using one or two sense resistors. You can choose between any of the following output current limit options:
1A
2A
2.5A
3A
3.5A
4.2A
Firmware Options:
Currently only guppy3drv is available. We will have Bistro available as soon as we get it 100% dialed in.
2. Bistro
I’m hoping the 20-22mm versions will have a bit higher 12v output and support both 1s/2s but either way ill still buy one of the 17mm’s as is I think.
Yes, I’m surprised the 12v is not a bit closer to half the 6v output.
We know that a bigger size driver can do 12v up to about 3A, but I’m not sure if Richards will go that high. He may dial it back a little to get better reliability.
This is just the beginning. More is to come. 
Richard is about to be flooded with boost driver orders once he lists them in stock 
Is there a reference you guys use to determine the proper output for XHP and MT-G2 LED’s. I mean, is there a good output to heat chart for each that shows how many amps it would take to blow an emitter?
I don’t want to be selecting 4.2 amps for a XHP50.2 when it can’t handle it. While I wouldn’t want 1A if it can really handle 4.0A either.
Check out these examples and look in each of their signatures. I like each member for different information so you should decide for yourself but they basically all test LEDs till they are almost or completely dead.
djozz
koef
Texas_Ace
It’s best to just Google the emitter name and specs. Switch to images and look for charts. I keep a whole folder full of these charts.
For a 6v 50.2 and MT-G2, 4.2A is fine for both.
Just so’s ya know, Richard has tested this boost driver of his to death. That’s the way he does things, he won’t release it until he’s very confident it will work reliably. And he’s at that point now. So while we’re just now seeing these he’s been torture testing for quite some time and tweaking to ensure they hold up.
I’ve talked to him about these quite a lot and of course am as eager as anyone to get an order placed. I’m told “Soon”, and I believe they’ll be worth it.
I just ordered one.
23 more available.