the 38mm works great, that's what I run.
when I first got my banshee it had trinity intake with a 35, fmf pipes, and a set of rad valves.It ran pretty good, also had 13/41 gearing.
switched to t-6 pipes, had more power everywhere.
added v-force reeds and 4 degrees timing, added more power again.
cut my head .025, still more bottom end.
added a 38mm keilhn, diden't hurt the bottom but helped the top.3-5 mph according to my gps.
needed a new topend so I set cyls to eric gorr and got the more power everywhere port, boonman lighten my flywhel (awesome mod), and a noss head with 19cc domes( awesome mod great customer service), and added a 15 tooth sprocket. now I have the power the way I like it, strong bottom, awesome midrange,and great topend pull.
That's my banshee and I love it
And remember a good holeshot and a ok bike will beat a fast bike and bad holeshot 9 out of 10 times.