I'm not sure I see the point of racing a stock M3. I would think a catless M3 with a tune, drop-in filter, and lighter wheels would be a better comparison. I own both cars, the M3 shown below and Stage 1 B8 S4 with GIAC tune, AWE Exhaust, and AWE intake. The S4 is faster, but the M3 is a much, much better car in all other aspects.
2014 White 911 C2S (On Order) | Leather Interior in Black | Sport Seats Plus | 7MT | PASM-Sport | Sport Chrono | PSE | Carrera S Wheels in Platinum | Bi-Xenon in Black w/ PDLS | Sport Design Front Fascia, Steering Wheel, & Mirrors | Burmester
2012 B8 Monsoon Grey Metallic Prestige Audi S4 | Titanium Package | AWE Exhaust w/ Resonated Downpipes | AWE S-Flo Intake | GIAC Performance Software