+1 on Yampols post.

I have the fmf powercore4 as well. I don't have the insert, but will try to get my hands on one to try it out. It's pretty noisy for no real gain.

2014 XT 250
-DynoJet PCFC
-FMF Powercore4
-Lowering Link (approximately 2")
-Fly Racing Bars