They are quite popular and seem to wake bikes up on the bottom end. I gave a lot of thought about trying one, but decided to go with an FCR-39 rather then a FCR-41 and a Power-Now. No way to know without trying one, but I think I will get just about the same results for less money. The two factors that made me decide against one, were the fact that IMHO, the LC4 already has plenty of grunt down low, and I noticed that it was advised that you drop down one main jet size. This lead me to believe it might slightly reduce airflow at higher engine rpms, and on an LC4 I felt more need for top end then more bottom end.
It seems to me these make a lot of sense on a MX or Hare Scrambles bike, where power output is becoming less and less of an issue, but throttle control seems to be paramount, but not so much so on a big honk'en D/S........... (Heck I've groundlooped my ADV a few times, by getting a bit more grunt then I needed for the available traction)
It would be nice to have some seat of the pants real world experience with one of these. So why don' you try it out for us?
PS: I know some guys on KTM talk installed them on their LC4's, and a large number of EXC owners as well.