I’ve got a set installed on my DH bike they do nothing to the shock itself, merely have the effect of making the effective eye to eye length shorter all you install is a pait of mounts where the hole through the centre is eccentric rather than concentric allowing you to cheat a few millimeters off at either end of the shock…
The difference it makes will all be down to the specific frame and it’s configuration, it’s worth taking your shock out and measuring how far beyond its normal fully compressed position the frame can go before components start contacting one-another in an unwanted way.
Doing it has had an effect on the ride of the bike, in combination with some other tweaks, it didn’t make a huge impact on it’s own really, certainly noticeable but not huge, it all depends on what you want from the bike really…
it will slightly slacken the Head Angle and Seat Angle, drop the BB an tiny amount and extend the wheelbase at the front by a weeny bit too… you can get a similar effect by running a little bit more sag in the shock, not quite the same but similar…
What bike are you considering it for?