howarmat

A tuned stang....meaning cai/tune and that is it is only gaining 25-30 HP generally with a good tune. It will still be a drivers race at that point....in this case, the GTO drivers sucks IMO because the OP has only added 10-15 or so HP over stock

acascianelli

With a K&N and an X-pipe (I'm assuming you still have cats), I'd say somewhere in the 275-280rwhp range. You really do need a better intake and tune. That GTO should have beaten you, either it was an automatic, or a bad driver.

Mods for more power...

Better intake/tune: A better intake and tune will easily give you 10 more hp, you should be around 25-30 extra hp from a good intake and tune combo:
UDP: These will give you about ~8hp, they will let your engine rev more freely.
Delete plates: These will give you another ~8hp.
Driveshaft
Longtube headers: expensive, but these should be good for another 15hp or so on a NA car.
O/R midpipe: Lossing the cats will pickup another 5-10hp.

But first, you really should get a better intake and tune.

1BAD3V

The 400hp 05 and 06 GTO's have the LS2, not the LS1 that you speak of. The LS1 GTO would take an *** whoopin by a CAI/tuned 3v. Stock for stock, I still would put my money on a 3v vs an LS1 GTO (aka fatass).