For emmision. I havent noticed any proformance differiences. I have noticed the boost gauge goes below vac 20 when letting off the gas and I still boost 6-7 in 2nd gear 86deg outside.
yeah I got mine done the other day. wasn't noticing any problems or anything but the stealership said I wouldn't have passed emissions without it. I haven't noticed any lack in performance or anything since tho so I dunno.