I currently own a Corvette 2002 Z06 and I don't think comparing both cars would be something possible nor comparing a M3 to a "2003" S4 would be possible or either the M5... Why? Not really the fact of the performance issue but I think that the S4 is different by all means. The S4 is all-wheel-drive and the M3 or M5 is rear wheel drive only. 4 doors for the S4 and 2 doors for the M3... Personnally, I'm planning on purchasing the "next generation" of S4 with the V8 just because I live in Montreal, Qc and the Z06 is sleeping in the garage 4-5 months a year and the S4 would be driveable 12 months a year wich would make a good compromise between a Z06 and a A4 3.0.
I have some sites with good infos on the S4 V8, I guess some of you already have them but here's a site with some "2003" S4 specifications.
I find 5.6 seconds for 0-60mph a bit slow, what do you guys think? A M3 pumping 333HP could do between something in the high 4 or low 5...