i have an aftermarket muffler welded to my stock piping. i was wondering if i should go to the shop and get some 2 1/2 in piping put on? would that help with performance because i heard that as long as the piping is stock, i can’t expect any gains from an aftermarket muffler. right now all i have is the muffler and dryer hose intake. i plan on goin turbo in the next few months as well. is 2 1/2 in. piping too big on a basically stock car?
there was a pretty good thread (i thought) about something similar to what you’re asking. it may point you in the right direction… (i got this off a search… hint/hint). 
thanx for the link, and the info was helpful but i’m not boosting yet. that won’t happen for a couple months atleast. i did a search and when it comes to bigger exhaust piping, most of the topics are for turbo’d cars. if having a better muffler on stock piping isn’t porducing anything, then i would think getting 2 1/2 in. piping would add something. or is it too large for a near stock car???
2.5" won’t be ideal for a stock engine but it’s not going to kill you. If your dead set on boosting then just go for the larger size. Depending on your power goals, you may also want to consider going even larger.
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Tom