None of the companies who make them provide info/dyno hp gains. I bought one a couple years ago to try out but it turns out you either have to modify (lengthen) your EGR tube or eliminate the EGR and tube, and I have to pass a visual smog test here in California so i didnt use it. There have been reports of 5hp gains, and no tune is needed. Mine is for sale.
pay attention to post # 9, 10, 16, 17 in the link they know what their talking about.
https://www.svtperformance.com/threads/throttle-body-spacer.794415/
As long as I don’t need to get re tuned then it’s worth it for me to gain 5rwhp and some torque down low to go with a manifold spacer. Probably would help to clean the runners while it’s off I’m guessing as well.
That was link for throttle body spacer btw.