It would be a huge achievement to deliver this weight. But power to weight would not be surprising at all.
I don't generally think Automobile Magazine is a good reference for solid rumors, perhaps others can clarify that claim.
1. The current EU spec 6MT E92 M3 has an unladen weight of 3649 lb (1655 kg) subtracting 220 lb from that is 3429 lb which is way over 3300 lb. Thus they have some basic facts incorrect here. The unladen weight of the F32 335i is 3329 lb. Either way sub 3300 lb seems very optimistic given the large number of components which must be
heavier in "upgrading" from the base model to the M model. It would take some very expensive materials and technology to overcome the certain weight gains (engine, suspension, brakes, wheels, tires, subframes, differential, transmission, etc.) This thread
has a variety of guesses and educated guesses on the cars weight and none get to (or sub) 3300.
2. Power to weight. Power to weight numbers would be reasonable with this very low weight and relatively lower power (than thus far rumored).
Current M3 Coupe: 8.81 lb/hp.
From article: 420 hp, 3300 lb: 7.86 lb/hp
Lets compare to the M5
F10 5: 7.66 lb/hp
E60 M5: 7.96 lb/hp
Only a 4% increase
Some reasonable F30 M3 power and weight guesses:
3500 lb, 450 hp
That is within 1% of the 7.86 lb/hp figure!
3600 lb, 450 hp
Thus the 7.86 figure of the article is definitely "in the ball park". I would certainly rather have the lighter, lower power solution as it would make it a much better car. I just don't see how they can achieve this without some really expensive (cost prohibitive) materials.
3. Power per liter. The current Porsche turbo has 500 hp from 3.8l. That is 132 hp/l. The GT-R has as 545 hp (stated, have not kept up as to how much if any the car is under rated. The initial GT-R was clearly under rated by about 50 hp) from the same 3.8l. That is 143 hp/l. 420 hp from 3.0l would be in between the two at 140 hp/l. Certainly not out of the realm of a production vehicle.
The great thing is that BMW absolutely can (and likely will, as the article mentions) wait to see where other competitors might be coming in in terms of power and weight and simply change a few lines of code, dialing in the desired (i.e. competition besting power level). Just like the M-DCT from the E92 M3 (and all subsequent DCTs...) the software is critically important.