In the comparisons Ive done, glasspacks don't give up any power at all vs any other muffler. Then again, if you avoid the louvers, the glasspack is just a straight pipe so they flow quite well.
Downside is they are LOUD.
If you compare power at the same decibel level, glasspacks will indeed lose. I like their sound and rumble at idle, if you can live with the noise. Our v8 truck runs dual long 2" glasspacks, very quiet and good enough as it only makes about 150hp per muffler.
