01-02's have higher HP #'s than the 98-00's. My Bro's 98 B4C M6 with lid, K&N and Flowmaster catback got 307 rwhp. A friends 98 SS M6 with lid, K&N, slp MAF and Loudmouth got 316. In comparison, another friend with an 01 T/A M6 with only a lid, K&N, and an open cutout got 337. My 02 Z28 M6 with lt's, ory, lm, lid and k&n, and maf hit 345 (i'm running rich and need a tune).

So yeah every car is gonna be different, even same year model and mods. But with yours I'd look for 320 +/-.

 

My '02 dyno'd 330 with only muffler, Lid, P&P TB (did the TB myself). Oh yeah, no screen and a smooth flow. I didn't get any stock numbers though. Anything over 300 and I would have been happy.