Are they even worth a damn? I know on carbed motors if you get a phenolic one it helps keep the carb cooler and helps atomization. Is it the same for tbi?
Basically, yes. Because the TBI injects fuel along with air into the intake, so it helps mix better. If you tried one on a '96+ Vortec, it wouldn't do a damn thing because the fuel is injected after the spacer.
On topic, honestly, there have been tons of stories of small improvements with spacers added. The title and subject of the thread should be changed to "If you wanna try it out, MAKE SURE you take your time and install everything correctly, and don't even bother on a '96 and newer engine."