Spacers only typically work well on TB or Carb apps where a central unit is on top of the plenum/manifold. On our styles where it's rammed in as is, they don't make much of a difference that I've seen.
I know this has nothing to do with the LS1, but I put one one my old 1999 GMC Z71(new body) with the 5.3 in it. I personally didn't see any noticable gains in performance or economy. I think the are a waste of $100 for straight induction like the LS1 and the 5.3 truck engine. But like CMNTMXR57 said, I have heard that they work alot better with Carb engines, or where the TB in located ontop. As for LS1 TB spacer, if they even make one, nah.
The TB spacers tend to be pretty popular for trucks but not with performance cars. I got slightly better throttle response using one in an 8.1 Silverado but it wasn't a light and day different. The drawback to some of these is the design produces an annoying whistle. There are only a couple that are quick (like the new Power Tower).