All M3's (all M cars in fact) to date have been n/a's, including the new M3 4.0ltr V8.
To be honest, we now have small cars with 5.0ltr-6.3ltr engines in them to get the bhp produced. I honestly think that we are getting to the point where, in this BHP competition between manufacturers will require smaller displacement engines using supercharging and turbo charging to get good strong power in future models. You can't simply get to the stage where such cars are kept n/a with 8ltr engines in them, its absurd.
BMW introduced the 335i using twin turbos as they had to keep up with other manufacturers whilst still trying to lead the way. Whilst I have not driven the 335i, I haven't heard anything negative regarding its power delivery or turbo technology.
Turbos also offer peak torque at very low revs and maintain that through to the top end. This makes cars effortlessly quick rather than having to wait for revs to build. Turbos and Superchargers also offer a wider scope to further tune performance without much hassle.
The one thing the M cars have always had, in my opinion, is a very precise and accurate feel, they are like precision tools ina class of their own (combined package of power delivery, response, chassis tuning, weight distribution, calibration of driver aids and sensors etc). Turbo charging will be fine, provided BMW calibrate devices to to retain a precision and accurate feel. Mercedes will probably use turbo's like sledge hammers as AMG's have always been the brute power house champions, so they will be able to get away with not being as precise with their models.