Folks have already took most of the the words out of my mouth...
As NISFAN pointed out in another post these specs do not look very plausible. The new engine will very likely redline in the 7500 rpm range (just a bit higher than the F10 M5). Similarly it should make constant torque to about 6500 rpm (the F10 M5 does so until almost 6000 rpm). However, 395 ft lb will make 420 hp at around only 5500 rpm. To not exceed a peak of 420 hp would require the torque to drop off steadily after 5000 rpm to only 315 ft lb at 7000 rpm along with a redline of 7000 rpm. Those simply do not seem like reasonable figures. If the engine makes torque and power curves anything like the M5 395 ft lb should make closer to 460 hp.
In short I do not at all buy 395 ft lb AND 420 hp.
Power to weight is what matters, period. Torque is relatively meaningless since you have to know gear ratios to make the torque something physically meaningful. That being said more torque does appear to lazy drivers who do not want to use their gearbox. With an 8 speed DCT this thing should maintain as high of an rpm as possible and take advantage of the DCT.
Weight claims: I also doubt they can achieve low 3400 lb range. It will just be too expensive to do so. Something in the 3500 range is more reasonable. However, it all depends on what the F30 335i obtains and I just have not seen a certain and final figure for that.
Power to Weight: 420 hp at 3415 lb would place the car right at about the F10 M5 power to weight although impressive, the current M3 has typically exceeded the prior generation M5 in power to weight. Something closer to 3550 lb with 450 hp is more reasonable solely from a trend perspective. Similarly there is still a power war of sorts going on. Most folks will just pass on the M3 if its power is so close or the same as the current generation car.
LOL at the comparisons between a tuned 335i and the next gen M3/4. Talk about not even a contest, strip nor track... This is almost a tautology isn't it. This is basically the same debate at tuned 335i > E92 M3 which only was the case under an aggressively tuned 335i and only at the drag strip.