Generally this is not true.
At 2A, the voltages are:
FC40 - 11.7V
XHP70 - 13V
XHP50.2 - 12.7V
XHP35 - 13.7V
4s LH351D - 12.9V
4s SST40 - 12.0V
Whereas 3s LH351D is only 9.7V.
Generally read datasheets.
Thats from FC60 datasheet, but im pretty sure its the same cause both rated same 12-14V.
Adding quotes since quadrupel seem to like to modify the posts after.
Design is completely by Tamagotchi, Quadrupel just copy and pretend that he designed it until he got caught. Even if you make your own pcb and layout (the easy part), the original design is still by Tamagotchi. He explicit stated it is not allowed for commercial use or sale without permission. If you look at schematic, the driver use Attiny85 for control of the switching regulator so it will not be compatible with Anduril or toykeeper firmware which does not have code to run switching regulator. It can be added but need a lot of work and is basically writing a new firmware. Quadrupel cannot even answer a simple question, which is expected since he did not design it.
I read the PM message between Quadrupel and id30209, and he clearly request for 12V at 1A output. Quadrupel say it can make it 'burst turbo like 30W but ill fix it on 20W for stability".
It is sad to see that you are rude to id30209. Everyone here is supportive of member trying to improve on designs and yet you do not change. id30209 clearly say he will be running a GT-FC40 led at 12V/4A, and you specifically said it will work at 12V output even if you didn't have equipment to test. You are trying to make excuse and imply id30209 is lying because FC40 is 14V led, but you are just making thing up. Clearly djozz testing show the led that id30209 is using is 12V or less at lower current. Even loneoceans lume-x1 with the same led is 12v at 2.75A, so it is clear the driver you sold to id30209 is not working at 12V or what you claim it to be.
It is a custom order by rgnwn, it looks like many people on the forum has tried it.
still trying to be snarky and mocking everyone.
You are just assuming and trying to push blame away from yourself when clearly the led is close to 12V operation at about 1 to 2A or less. Even the datasheet for fc60 show 12V at 1A. the soldering quality also look very poor, very different from the photo you post in this thread.
for id30209, maybe you can consider using other driver which are designed by actual members that work instead of ripoff. Mtnelectronic has 20mm boost driver but out of stock http://www.mtnelectronics.com/index.php?route=product/product&path=67_118&product_id=959. loneoceans lume x1 seem to work well. Convoy also seem to have 12V 2.3A single cell driver https://www.aliexpress.com/item/4000046041955.html.
clientequator is that you ?
For record
Time to be out of here as well! I hope Quadrupel can learn from this experience.
“So make your own then”
What an informative thread. Highly recommend. 
It’s like coming across the scene of a few flares burning out their last ounce of fuel before going out… little bits of plastic and glass scattered about the roadway… and nothing else to see. You know something bad happened here, but there’s just not enough clues left behind to even begin to understand it. 