That kit is a pretty good value, all things considered. But you'll lose about 9cc's (240 vs. 249cc's) and it will produce less low end torque due to the TW's shorter stroke. It will work on TW cases if you open them up to XT/TTR spec.
I've talked with them about it. It's a proprietary kit with a new, thicker liner and they've sourced their own pistons. It's pretty hard to find a 74mm piston with the same piston pin size as the TW/TTR/XT. They're pretty much a cu$tom propo$ition.