The idle is fine. Very deep and a little on the quiet side. This exhaust doesn't pic up tremendous volume until you are over 3k rpms, then it really growls. Full throttle just screams.
Muffler-less sounds good to some folks. It is louder, raspier and less refined than with a bullet. It's a matter of taste.
I saw your post asking about back pressure. The need for back pressure is a myth. Pressure is just something your engine has to waste energy overcoming. Either way you go, !Muffler or Dynomouth, you will have the same back pressure... The bullet muffler is 3" open pipe straight through. No fins, louvers, chambers, etc... to disrupt flow.
Just going seat of the pants, the car may have lost a touch of torque, but it revs quicker and runs hard in the LS1s natural power range. My dyno numbers are 318 rwhp & 334 rwtq with just a lid (paper filter), Dynomouth, and a couple of free mods that don't generally show on the dyno. This from a 98, most of which run about 280 rwhp stock.
Good luck
Dave