I have a homemade version on a Martin ShadowCat with a shoot-through cable system. In other words I have hole on the riser where the cable guard rod would have been, and it wasn't being used. It fits right between the cables with no problems, and it contacts the string right at the top of my center serving closer to the middle of the string than most I've seen.

I'm one of those that doesn't seem to be able to tell a difference. The bow does shoot different with it, but that would just be a matter of retuning the bow. I did not chrono with it so cannot comment on speed issues.

I do honestly believe that some bows can benefit from it. Just which ones is a matter for each individual to determine. Each setup will vary.