This is pretty much my opinion, I've had the SLP loudmouth, flowmaster, and Magnaflow all on my car at differnet points. I put the Magnaflow on Wagner's car with no other mods and it wasn't much louder then stock at all, with headers his car has a noticable sound. My car currently has a Magnaflow on it and with a cam and headers it has a nice tone, mildly loud, sounds good when in the gas. SLP wasn't really too loud until I had the headers and cam in it, but at that point it's just ridiculas. I think the Borla on Chad's car sounds good, but not enough to buy it.
I think the lid is subjective, didn't notice much, a little more sucking sound. A good difference with more mods. LS1s love a free flowing exhaust so a nice set of longtubes (shorties don't flow more then the manifolds), good y-pipe, and a catback of your choosing will go a long way. At that point short of a tune the car will need something other then basic bolt-ons to make it worth the money spent in my opinion.
Tie the frame, it made a huge difference in how well the car feels.