In other words when doing fresh builds do you need to start out with a bought new, known source/known bin to get the performance and be able to push the emitter? Or are XM-L2’s you pull out of lights just as good as store bought new ones as long as your ok with the tint?
Obviously there’s a difference between T6’s and U2’s but if I’m cool with that is there any other issues from reusing the old emitters? Are they not as good, or not able to handle the heat/current as well (I’d reflow them onto Noctigon’s obviously)?
I’ve recently gotten lucky and gotten 6 pull XM-L2’s, is there any reason not to just use them?
If they're different and you wire them in parallel and throw a ton of current at them there can be enough difference between their Vfs that you can have problems. In theory, anyway. If they're all known parts they'll be closely matched enough that won't be a problem. Wired in series it also won't be a problem, even if mismatched.
Thermal runaway. If they're all in parallel, and one has a Vf of 3.1 at 3A, and all the others have a Vf of 3.25 at 3A, then the one with lower Vf will take more of the current than the others, possibly enough to pop.
As a very extreme example, hook up one MTG2 in parallel with one XML, then feed it 6 volts. You can guess what would happen, even if the current were limited to a level the XML would otherwise not have a problem with.
Use 'em! Even if you get into thermal runaway, what are you out? You got the emitters "for free" and you can reuse the copper MCPCB. I guess time is a consideration, but honestly as long as they're all XM-L2s on the same base I wouldn't worry about it too much.
Do you think that these budget manufacturers take the time to carefully test and match the vF of these emitters in the multi-cell parallel lights?
No, they don't, but when they all have come off the same reel there's little mismatch possible. Even if they switch reels in the middle of a run and the two reels happen to be different, how many lights would end up affected?