We're talking about a N/A engine from Audi and a N/A engine from BMW - near same displacement (0.2dl more on the RS4.) The BMW engine will be significantly better. As evidenced by the 5.2 liter V10 on the S6 and S8 Audi's only outputs 320-331kW and is much newer than the 5.0 liter S85B50 on M5/M6 that outputs 373kW. In M3's case the BMW engine will be newer of the two... BMW will need to have the M3 completely obliterate the RS4, that should be obvious. That's the only competitor in it's segment. This is probably why BMW will want to get the new V8 up to 9000rpm, and might still be able to pull it off if they want to. It might postpone the car, but if they want to do it, they'll do it. The RS4 revs up to 8k with only 420hp DIN and 430Nm of torque. All information aside, the M3 will have to put out at least 450/450 of each to obliterate the RS4. And as calculated here several times, it's easily doable, especially with BMW's DI technology.
In the S6, the 5.2 liter V10 (copied from BMW's S85B50 idea directly) takes the S6 from 0-100 in 5.2s regardless of the AWD... The M5 which would be the direct competition there, is 4.7s 0-100 with RWD only. On dry conditions, the results are clear as a day, the S6 doesn't stand a chance. The S8 is 5.1s 0-100. Part of the poor times of the Audis have to do with the fact that they still have to use slushboxes, as they don't have any SMG-type technology for their gearboxes, and DSG doesn't work for over 300Nm of torque (it'll blow up), which slows them down, but not that much. Their V10 is just powerless and doesn't rev as well. The RS4 V8 will seem powerless and non-revving compared to the new M3 V8 as well.
RS4 does 0-100 in 4.8 seconds, so in that field, it won't be hard to decimate on straight line even if the BMW engine remains at about the same figures. But 450hp (DIN!) is a number I've heard so constantly that I won't believe it only when I see the real final numbers from BMW.