Yeah, you may have more brightness with lithiums over NiMHs, but it should be extremely little difference. I found that the converter board in the ProPolys are current regulated (I believe), but the exact current level seems to vary from light to light (may depend on the bin of the Luxeon). I found that the current going to the Luxeon in my light was around 500mA. If you want really good brightnesses and run-times, use the Seoul in there and cut down on the current, and then use the lithiums. That would be nice, but the emitter swap and current change would be fine alone (for normal use without extreme environments). If you want to learn how to do such a mod, I have a thread where a Seoul replaced the original Luxeon and I found how to vary the current level. I do not want to post the link because I would feel as though I would be taking over this thread, or something. You can PM me, though (or just search the LED section). Good luck with that, though. I plan on using a Downboy converter in mine, so if I need to, I can use rechargeable lithiums in there, also...
-Tony