I had HRE P40's for a while in their recommended sizes, and personally felt there was a performance DECREASE. Even though the wheels may be similar weights, the additional weight you have to carry around with the wider tires hurts acceleration. While we have plenty of HP, we do not have excess TQ and 295's in the rear are overkill and are a hinderance.
IMO, BMW did such a great job on the OEM 19's , I actually like the look better then the P40's. The OEM 19's are really nice, with an almost 3 dimensional look to them, and they are forged and lightweight.
IMO, OEM 19's FTW
16' M3 Mineral Gray, DCT, Adaptive M sus., 19" wheels
15' Audi S4
13' Audi TTRS (APR stage 1, MSS springs)
09' C6 Z06
08' M3 Interlagos Blue: 6sp, Tech. & Premium Package