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.
On Order: 2015 Mineral White M3
| Sakhir Orange/Black Extended Merino Leather | 6MT | 19" Black 437M Wheels | Harmon Kardon
2014 White Carrera S
: Money2536's White 991 C2S Journal