If it feels faster than who cares
Did twice pipes help my stock Z? Probably not...probably hurt it in lower RPM's if anything. Does it sound good and make me happy? Hell yes it does!
Anyways, take what JohnC said and do the tuneup. It sure helped my Z. Unless you bore the engine out, change fuel management, or gut your interior, nothing will really do a whole lot for a N/A L28.
Just my $.02