Hay, I hadn't never heard of that about losing a few horsepower and torque if you go to big.
I had a Camaro that I put the Flowmaster 40 series muffler on that gave me the same setup that the Z28 and SS had a quasi-dual exhaust. Meineke put them on for me welded everything up just perfect and I didn't have a problem with them except the tips I had chose where polish not stainless and eventually started to rust. I had to go back and get some stainless steel ones for the camaro.
I can say that it doesn't make the car sound like a bee or sound ricy. I didn't have any resonators on my camaro and it sounded really close to a somewhat stock Z28... So much so that after I started driving around town and just enjoying my exhaust setup on the camaro, people would stop me and as if that was a Z28 or they hear me come up in a gas station and ask the same question. So I can say that flowmaster mufflers will give you a good sound, unlike the magnaflow they produce an excellent throaty sound, magnaflow mufflers sound a little crappy (no offense Mario Andretti people out there). For those GT owners here that say that the V6 will never sound like a GT they are wrong to a degree.
The only time they are right is when you rev to high rpms, no matter what you do to the exhaust at high rpms (4200+) you get a really cool racy like sound, have you ever heard a V10 8.0 Liter Viper GTS rev, its so cool. At low rpms it as deep as a V8 but with it own distintive sound, at high rpms it sounds like a race car (to a degree). Same can be said about the Mustang V6 and Camaro V6. A GT or any V8 for that matter be it from Ford or Mercedes Benz SL500, as it revs higher it gets deeper and more throaty. A V6 or V10 or even V12 will get racier and racier.
Hope that helped some...