I've had an '09 with the Premium Package (V6) and now have a '12 with the Premium and Tech packages (again with the V6), both purchased used. Look for deals! If you live where it snows, no one wants a high power RWD car on their lot. cargurus.com shows you where the best deals are in your area. Also, with the 2015 being a redesign, 09-14's can be had on the cheap(er).
There is a good bit of difference performance wise between the 333hp 8 speed and the 290hp 6 speed. From what I've read, there's not a ton of performance to be had with an '11 V8 that you don't have with the '12 V6. (Both are low-to-mid 5 second-to-60, high 13-second 1/4 mile cars.)
You might want to double-check this, but I think once you upgrade beyond the basic trim, you get the 14 speaker Lexicon stereo-- which is upgraded to the 17 speaker system when you have both the Premium and Tech packages. I don't think, in either case, you're stuck with the stock 7-speaker base unit.