BMW will have no problem selling the new M3 because the M3 is an exclusive car. People will buy the new M3 simply because it's an M3.
Is the 335i upgradable to match the performance of the new M3?
Yes, I'm sure it is!....BUT,
All the mods needed to upgrade the 335 to the performance of the new M3 (track and drag) will cost plenty of money and at the end of the day...you might as well buy the new M3 (assuming the MSRP will start around $55K as predicted!).
Procede, LSD, Full Exhaust, Intake, Suspensions etc...
So you have $5300 dollars in mods right there....
An M3 will always be an M3!!