If you have a tune with one type of turbo back exhaust, and then get another type do you need a re-tune? My initial Turbo back had a HKS down pipe with OEM type flange. I have changed out to a full APS turbo back with divorced plate down pipe. With the possible air flow difference, do you need a re-tune. I had Jeff tune my Subie. Any opinions would help.

EDIT: I'm not having any driving issues, but it seems to surge a little under WOT.