Did you check the Oring seal for the TB? I have R&R'd my TB several times and always find some gunk between the SC and the TB. You can also use an unlit propane torch to go around anywhere you think there may be a leak. Use a piece of hose to attach to the end of the torch get into areas you can't get the torch. Go around the composite manifold as well, I know there were issues with the composite manifolds on some of the LS series engines and the Vortec's as well. When the idle goes up you have found the area. Some people like to use carb cleaner or brake cleaner as well. I prefer propane, to each his own.
Jeff
