I have a XM-L2 U2 here, but its on aluminium… I put it in place of the U3 and got 4.13A 
what the hell? That much of a difference??
Yes. In order to better understand how this works, please take a look at the XM-L2 graph djozz provided here:
(post #51)
Either graph will do, but the XM-L2 T6 graph is the bottom one. As you can see, every 0.1v is accounting for approximately 0.5A of current. Your linear driver will limit current as intended only when Vf is low enough to give it at least 0.12v of headroom above the Vf at the desired drive current.
Now you just have to learn how to reflow from the aluminum board to the copper one. Yet another place we’ve all wasted a few LEDs.
What tint are you looking for?
Well I guess that’s why I’m getting 4.1A now 
is 4.1A with a U2 going to be noticeably brighter than a U3 at 3.2A though?
Tint doesn’t really worry me, as long as it isn’t blue like you get with cheap aliexpress lights 
Yeah I’ve already wrecked one, I don’t have any solder paste which doesn’t help!
Meh. Probably not really. You need big differences in output to notice it. But ~3Amps on an XM-L2 U2 is actually sustainable for a little while since it doesn’t start below the minimum input voltage for regulation. ~3Amps on your XM-L2 U3 is with the driver already out of regulation - therefore it will fall continuously. That’s why I’d want the lower Vf part.
Solder paste does help but before I had that it helped to pretin the mcpcb pads and use the same method, heat it up until the led slides into place. Practice on the dead one. The copper mcpcb will take more heat than the aluminum one to reflow.
Well based on that I think I’ll drive a XM-L2 U2 at 4.2A in one C8 and put a U3 in my other (2.8A) C8.
No, that’s just flux. Solder paste is gray. It has has tiny tiny balls of solder mixed with flux.
If you’re in the usa RMM / mtnelectronics.com has solder paste and ships fast. Fasttech also carries it.
There’s rosin(a type of flux), solder, and solder paste. The paste is a mix of flux and ground solder. That link is for flux and is what is used with wire solder.
FT solder paste http://www.fasttech.com/products/0/10003546/1261003-chips-repair-tool-soldering-paste-grease
I use kester but there are plenty of other options that I don’t have experience with.
Right thanks, so that Fasttech stuff will work well for reflowing?
Is solder paste better than solder for reflowing?
It should work fine, but not having it in a syringe is a pain and will guarantee a lot of extra effort.
This is the exact product which I use: Kester EP256 Solder Paste, No-Clean 63/37, Syringe Dispenser
Sorry, I forgot that the conversation was about reflowing emitters, not drivers. The little tub is probably OK for that. I have trouble getting the right mount of solder on the pads, even using a stencil (Any interest in a CREE LED solder paste stencil?), it doesn’t matter whether it comes out of a syringe or a tub in that case.
Before I started using solder paste and stencils for emitters I often ended up pre tinning the star pads and sanding off the excess as it was very difficult for me to get the right amount of wire solder in place. Still, most members into modding have most likely done more of this more frequently than I so I remain somewhat unskilled at this. I like stencils and paste.
Looks like this will do the job then? http://www.mtnelectronics.com/index.php?route=product/product&path=25_91&product_id=255
Sounds like yes.