My brother has a modded 01 S4 (ECU-Chip, full-exhaust, intake, stoptech brakes, carbon-fiber hood, small battery, wheels/tires etc). He is making 260HP/320trq. at wheels according to the AWD dyno. As far as speed we are dead even when we race.
The S4 is a bit heavier and feels heavier than the STi (less tossable). The steering feels less precise and the gears on the tranny seems alot longer and not setup to keep it in the powerband like the STi. The extra weight that the S4 has over the STi is for luxury and sound-deadening materials (which the STi really doesnt have). Also, for longer rides the S4 is much better and more comfortable, however, the STi is not in anyway unbearable. My brother spent $10K on mods in his S4 and it is not any faster nor can it turn quicker or out handle my stock STi. That says alot about the STi. If you can deal w/out having the current luxury of your S4 then the STi is for you. :lol: