At 10% of max current you get of highest cree bins 200-210 lumens
At 33% you get 170-180 lumens/watt
At 50% 150-160 l/W
At 100% 120 l/W
So for 400 OTF light you need for an XPL HD V6 about 1A when calculating 20% optical losses
So you look at pretty much 3W heat from a CC linear driver and LED, this is no problem for an 1 inch diameter light
Going for a triple LED light ght would reduce the heat about 10-15% which isnt worth it
A triple runs at one third the current to each emitter so bin efficiency is maximized. Unless you want a thrower a triple is definitely worth it to maximize output and minimize heat at a given current.
The difference between 11 and 33% to reach 400 lumens from a high bin XPL HD is about 12.5%
If he wants 1100 lumens then a triple is up to 40% more efficient
TIR lenses are less efficient as a refkector with AR glass
Often triples on small hosts loose lumens because the bezel blocks some area of the TIR lens, so the better efficiency is lost again
I can vouch for that, my triples generally put out about the same lumens as a single LED at the same current in the sub ~3A range. Obviously as you go higher the triple pulls away.
I can compare the xp-g2 and the XP-L HI from the Jetbeam 1mk and IImk, the XP-L HI runs a lot cooler at max mode while putting out more lumens compared to the xp-g2. The IImk also manages to remain cooler itself and keep it’s cell cooler while on continuously.
Hello, OP here.
My S2+ is 6x7135 so yes, it is too hot to handle after like 5 minutes on.
This is a lot of discussion to the topic. However, I admit that it’s still confusing for me. If it is lumen per watt, then the bigger the better? what about Amp and Volt? I might have to dig deeper in this field. Normally I just look at the lumens and brand.
I think I need to find bigger lights for 400 lumens then.
And thanks everyone for the answers 
Bigger is better for heat and focus since a larger head will have a lower equilibrium temp than a smaller one and larger, deeper reflectors capture and focus more of the emitted light. Understanding Ohm’s Law will take you a long way in understanding V, A, I, R, and P, and how they mix to affect light output.
The efficiency of a XPL HI V2 is slightly worse than an XPG2 S4
Only a XPL HD V6 is 100 lumens brighter at 1.5A
The reason your XPL HI seems brighter is that it is a bit more focused and the original XP-G2 was not a high flux bin
In the Jet-I and the Jet-II, am pretty sure the amp draw on high is different, explaining the differing brightness (with the Jet-II XP-L HI being clearly brighter) so it was my bad to assume that was due to better efficiency. Nonetheless, to answer the OP’s question, the Jet-II stays manageably warm while on a continuous max run. I don’t have S2/S2+ to compare the heat, though.