Put MBRP on my 2012, from WOT to 3/4 throttle it sputters and wants to shut off for a split second. This is in the 6200 rpm range. Put factory exhaust back on and works great. I'm gonna run factory exhaust till I find something else I guess.
Not sure what your problem is but sounds exactly like my 2011 800 etec. after WOT and drop to 3/4 throttle it would sputter around 6200 rpm, even shutting down for a split second. This caused major troubles when carving hard if you dropped throttle and it would stop quickly then keep moving...