I'm a big fan of the 40 series my self.
I always tell people - make a decision on the exhaust based on how your going to be driving the car.
If your driving the car back and fourth to work everyday 40 miles each way... Flowmaster may not be the best option, as the drone can annoy you after awhile.
Than Corsa's RSC technology becomes a great option.
If it's a car that you could careless about drone, noise etc... Than you have a ton of options
Your best option is to just browse YouTube etc. and find which sound you like the best.
Don't be scared of Flowmasters and "losing" power. Never seen it, I actually ran a dyno test between Flowmasters and Magnaflow's on the same car @ 450 RWHP. Car made the exact same horsepower, give or take 1 HP. With the amount of variables, I'd say there is very little difference.
All the 2 1/2" Cat-Backs seems to make the same power.