Not real fond of Flows myself. Flowmaster mufflers are notorious for a part throttle drone. Every car I've ridden in that had Flows had that drone and it doesn't take long at all to really get on my nerves. To be completely honest, I've never ridden in an S197 with the American Thunder system, so I can't say for a fact that system has the same drone, but based on past experience I'll never consider any Flow system for any of my cars.
Considering your car is nearly stock, you really don't need a cat back system. The oem pipes are more than adequate for the 300+ hp of these cars so an axle back system (basically just the mufflers and tips) is all you really need and you can make a big difference in the sound with the right mufflers. I'd check out magnaflow mufflers, or go for FRPP FR500S mufflers. You could also try finding some GT500 take offs for a sublte increase in tone and volume over stock GT mufflers, but they are too pricey to buy new IMO.