Bench racing FTMFW!
No one knows what the new M3 will drive like yet. I agree it would be nice to have a little more torque (it always is) but, I'm sure the engine response of that motor will be awesome especially up top. Which is where the 335 motor is kind of flat with the stock turbos. Once there are some after market turbo choices I'm sure the 335 will be able to trounce the M3 on the top end but, we are longer talking stock for stock at that point.
Regarding handling I'm sure the M3 will be much more nimble and firm than the 335i. Again I'm sure there will be after market solutions for this as well. The same with braking.
I think the M3 will be hard to get appreciable gains out of not to mention incredibly expensive to mod. Lets face it you can make modding a 335 pretty damn expensive if you want to as well. The $8K supersprint turbo back is proof of that.
Now you've come to a point where you probably have better performance than the M3 but, have pretty much rendered your warranty null and void and spent as much or more money. So, what's that worth?
I really don't think BMW will have a hard time moving M3's. I think it will be considerably quicker than an RS4 given how heavy those car are and their quattro systems higher parasitic losses. It's not like competition wasn't already out there. They knew what they were up against.