I have scientific data Although I do not remember exact values and do not currently have access to the data.
I tested both noise level and speed with Bowjax Slipjax string dampeners with my Onza 3 as part of my Hoyt CRX comparison test. Shooting noise was reduced by several decibels but speed was not reduced at all (measured the speed before & after). You could CLEARLY hear the difference in shooting noise and the precision decibel meter confirmed this. I installed the Slipjax dampeners as close to the cams as possible.
I later attached Bowjax Monsterjax dampeners to the limbs, but I did not make any measurements and it was hard to detect any difference since the string silencers had already done their job so well. The string silencers made a huge difference, anyway. I highly recommend them.
Rytera Nemesis, 2012
Martin Onza 3, 2011
Hoyt CRX 32, 2011 (SOLD)
Martin Phantom II, 2003