Well, to your first question there is a big difference between a manual and an automatic - one you have to shift on the time the other you don't. But if you mean performance wise, then there likely won't be much of a difference. I would go with your gut on what you buy. Don't listen to others because they don't know your complete situation. And even with the auto you will have the paddle shifters that will help you feel like it is a manual if you want.
On your second question, yes. GMPP already has a nice lineup of accessories that should add some power to the car.
http://www.gmperformanceparts.com/se...o_sitelet.html Once the car is out, there should be lots of other aftermarket options to add power too.