atleast go 2.25" both sides, even that would loose you a lot of back pressure
as for duel exhaust... it's ganna coz a fair penny to get it down the left hand side of the car! Ask around, and if the exhaust shops have done it before, coz there is a lot of stuffing around.... you might wanna hold off those plans for chip before you get your exhaust

maybe consider getting a cunky cam instead so it can provide more exhaust flow