well heres my .02cents:
the next M3 will almost for sure be modelled after the E46 M3 CSL and the M6. BMW has been experimenting with light-weight materials, and will push the enevelope even farther with the E90M3, thats for sure. They've already perfected the use of those new smc ultra light bumpers which will be on the E90. They will almost for sure have a lightweight CF roof section. The big question is willl they have the guts to put the superlightweight hood/front fender/trunk sections in? Or just put in a relatively lightweigh aluminum piece like on the E46m3. With all this combined, I can bet you the next M3 will be lighter then the E46m3, even with the larger (but not much heavier) V8 derived from the M5/M6 V10 in it.
SO the car will be a bit lighter, with a whole lot more power, and more importantly more TORQUE. the extra torque is really going to kick the 0-60 time down a good amount..
I honestly believe the competition is going to have something to worry about again with the next M3, just because of the brand new engine BMW commited too and the long term comitment BMW has to the use of lightweight materials. They've already tooled up their factories for large volume production of the smc body parts.
lets look at the e46m3 for an example
the stock M3 weighs : 3400lbs.. the M3 CSL weighs... 3050lbs..
thats a nice chunk of weight savings
the stock M3 has : 340hp the M3 CSL has : 360hp
now that extra power + weight savings translates to this change in 0-60 time: stock M3 : 5.1secs M3 CSL : 4.8 seconds
thats not bad, I'm going to make a blind guess and say that a stock E90M3 with all the electronic gizmos will weigh in at about 3200lbs and will make 400hp with 320ft.lbs of torque. This should all translate to a 0-60 time of about 4.6-4.8seconds would be my guess.