Mazda is marketing the Mazda6 as a Sports Sedan, which if I'm not mistaken, is equivalent to a Sports Saloon in Europe. Sports Sedans/Saloons are usually high power grand touring cars. If there is any car in the Mazda lineup that needed a higher performing engine, it is the Mazda6. I'm not surprised that the Mazda6 is the first car other than the CX-9 to get the turbo engine.
Now regarding AWD and FWD. Like I said previously, I'm fine if it is just FWD as long as it comes with a LSD. I'm a big fan of AWD, but FWD + LSD should not be underestimated. The Gen1 and I think also the Gen2 Mazdaspeed3, was faster around VIR than a competing WRX back then. The current generation GTI is pretty quick too and it only has FWD. Sure, an AWD car will have an advantage in traction, but that is something that can be mitigated with stickier tries and better throttle control. I do agree that making AWD available for the Mazda6 would give it one more desired feature that other midsize sedans don't have, most likely helping the car sell more as well.
I'm getting above average fuel economy with my Speed3. I'm getting 22-23 average mpg on my current commute, on a car that's rated 25 mpg on the highway, not too bad. I used to get 23-25 mpg on my longer commute in the past. As long as the tuning is not too aggressive on the new engine, by that I mean it doesn't produce too much boost/power at very low RPMs, good fuel economy should still be attainable with the 2.5T engine.