They're a complete waste of $$$. And the graphs in Jp Magazine's showed so much latitude for error it wasn't even funny, that 1hp they thought they got with the spacer couldn't accurately be determined due to the normal error rates and the eratic charting they did. It was all hype.
Spacers do wonders when underneath a carburetor or sometimes a TBI (throttle body injection) where there is an air-fuel mixture passing through the spacer. But the TJ's throttle body and any spacer present only has air passing through it, the fuel is injected well below directly into the combustion chamber.
Save your $$$. Have I tried a TB spacer? Yep, one of the fancy red anodized Helix spacers and it did nada. I couldn't tell any difference after I installed it and I could tell any difference after I removed it due to its whistling. It's sitting in my junkbox, I won't even take the effort to sell it since I wouldn't try to claim it does anything.