I have noticed when LT's were added to the exhaust, that it sounded much "cleaner". That is from 2 friends that have added it to their borla and magnaflow atleast. I think it sounds better IMO.
JOHN

 

The really BIG difference I got in loudness was deleting the CAT. My car's damn near ungodly loud. But, I only drive it on weekends and occasionally during the week, so the UMPH is much more important than the comfort of driving in silence for me.

I agree with whoever said the headers make the car sound much cleaner. It gives it a more defined, and maybe refined sound. And, so many people out there luvin' Borla...I HATE the way Borla sounds...YUCK! Especially on an LS1...A$$ is what that sounds like.

 

Well, let's see here... A stock throttle body with an airfoil will flow over 600cfm (from what I've read). This is good enough to feed a 350cid engine up to 6000rpm at 100% VE (volumetric efficiency). Plus, the ZZ502 (fuel injected version) makes over 500hp with a stock L98 throttle body.
With that being said, if you increase your displacement, rpms, or VE (heads, cam, exhaust, etc.), the bigger TB will benefit you. I have a book called "Small Block Chevy Performance" by Dave Emanuel that has a series of dyno tests on a Camaro LT1. They dynoed the engine with ported heads by CNC Cylinder Heads, CAI, and a 52mm TB from TPIS; and gained 12hp and 6ft-lbs of torque. Everything else on the engine was stock.