That's how these cars are no matter what the exhaust setup. The idle sound is cool, but once you get over 1500 rpms they just sound......well....not so good.
I had Flowmaster 40s on my last Monte, but got tired of hearing them while cruising after about a month. Sure, the startup/idle sound was great but actually driving got annoying. I got on the highway once and realized that my car and the beat up Honda with a loud exhaust next to me were droning at the same exhaust note. It was like bad rasp in surround sound.

To each his own, but IMHO: Stock mufflers FTW!!!!!

-Riggs.