M2 is still an S6 with a turbo, going to be similar to M4 but with less torque. The M2 comes back by being a little less weight and a more focused car, so you may like it for that.
Remember Mustang GT PP is Fords just run of the mill Mustang, the one they sell huge volumes off. This is why in a way its upsetting but also good reviewers compare it against far more expensive BMW's, Jaguars etc.
If the GT 350 was available in UK, I'd have one and it would be far more interesting to see reviews of the 350 vs M4, F-type and other stuff because the 350 is the true competitor to such cars, not the GT.
Also the GT500 is rumoured for 2018, expect GT350 handling but a lot more power no doubt using the 5.0 engine though.
To answer your question a supercharged Mustang GT is beyond crazy, as long as you go with a roots style blower, the ones that develop huge torque down low, makes them great fun and very peaky. Roush/Ford or Whipple is the best bet for super charger, as huge torque, insane power but reliable and true bolt-on that you can do yourself easily.
Couple of videos to give you an idea of how fast a Mustang is with a Roush or Whipple super charger, engines are stock, transmissions are stock so this is $6000-$8000 worth of supercharger kit.
Though I have complained about how the stock Mustang GT PP feels a bit soft in the rear, don't think of this as its a poor handling car, afterall it and the M4 are lapping circuits within 1s of each other, it is by no means slow on a track:
You can see in the corners it is very stable and has great speed, it is only lacking in power as that above is a completely stock car even down to the Pzero tyres.
