One of the mags got 11-15HP (depending on RPM) to the wheels out of a stock GT with only the SLP catted X...its like $379 if I am not mistaken (sp?).The catback added like 8HP...I belive peak gains were 19.5 with the X and SLP catback.I just dont know about $700 or so for 20HP...hard decision, think the X pipe is next for me though...when I hit 10K miles.
That's a very generous amount. I'd bet most aren't seeing more than 5-7hp with an O/R H/X-pipe and maybe another 4-6hp with the cat-back. Stock cars don't greatly benefit from exhaust only mods.