I've tried those speed balls years ago, and Martin speed demons, puffs, String leech, etc. Put them at various locations on string between 2" to 5" from cam/wheel, which did have some impact, but still all lost speed regardless of claims..........and I always went back to cat whiskers.

Last year after some tuning, I shot my Bengal thru my chronograph prior to putting cat whiskers on, then again after install of cat whiskers at about 3" from cam/wheel. 8-10 shot average, I lost 2fps. Some years back, I had about same loss on my '96 Firecat.

I put them on very similar to this video http://www.youtube.com/watch?v=Z6WNxTLPJW0