Just because a muffler sounds loud doesn't it mean it flows well. It can also mean it is a low quality muffler makes a bunch of back pressure and doesn't muffle sound very well. furthermore, a loud sounding engine can seem faster when it really isn't.
Did you look at the web site that I listed before?
2-1/4 glass pack flows 133 cfm and is loud as hell
2-1/4 stock OEM muffler flows at 149 cfm and is very very quiet.
2-1/2 dynomax super turbo flows 268 cfm and is also very quiet.
So which is the better muffler?
A loudass flowmaster doesn't even flow as much as a quiet dynomax super turbo.
So in theory, if you use dual 2-1/4 glasspacks for a total of 266 cfm, it would have enough flow for a 120 hp engine (2.2 cfm/hp for zero loss). That is, after 120hp, the dual glass packs start killing power. Twin dynomax's start losing power after 240hp.