I would easily pick the Mazda 6 over the Aston Martin-ripoff front-end, confusing array of buttons and gadgets, and heavier Ford Fusion. Sure, there are a few different engine options and the Fusion isn't shy toward a bit of performance-oriented driving (my choice would be the 1.6L turbo with six-speed manual transmission), but it just seems excessive all-around to me - the weight (which means it NEEDS more power), the technology, the dimensions, and more.
Although I'm happy with my Sky 3, if I were in the market for the 6 sedan, I'd hold out for the availability of the six-speed manual transmission in the Touring trim later this year. Hell, if Mazda decides to add at least a Bluetooth option to the Sport (my manual 3i Touring even has it), that would be more than enough for me as the Touring adds "leatherette" seats and tech options I have no business using behind the wheel. Another thing to consider would be waiting until later this year when the 170 HP/310 TQ diesel hits our shores. Just a consideration.
As far as performance enhancement is concerned, the 2.5L Skyactiv motor has just launched in the US market for 2013. As the above poster said, it's much too new for strong aftermarket support (it'll build for sure), but I wouldn't readily expect forced induction options due to the higher compression ratios of these engines. As for the Sky 3 hatch I have currently, although it is stock, I do have a couple options at my disposal. First of all, the Bridgestone Turanza tires need to be ditched immediately for a better set. Secondly, Corksport has already designed and made available a short-ram air intake and aftermarket exhaust for vehicles that have the 2.0L Skyactiv motor like mine. Sure, it could add a few horses or whatever, but fuel economy will actually increase under normal driving conditions (I can't wait to get this done). There may be some more options on the market now (these became available in 2012), but at least the 2.0L 3 and CX-5 are starting to be covered.
Good luck on the car search!