Recoil reduction will depend on what cal you are testing. While it will be dramatically reduced, save your shekels for a new scope and maybe some stock repair. I've seen the dreaded Sled shear off base screws and bugger up more than one scope. Better and cheaper to learn how to manage the recoil than to use that gimmicky tool.
We have encountered this many times in warranty claims on Leupold optics. All of that energy has to go somewhere - if the rifle is unable to move, the force is transferred to the scope and mounts. The effects are increased with some of the massive, overly-heavy offshore optics; all that mass will negatively affect your base screws.
A better option (when possible) is a muzzle brake, if you are having difficulty with the recoil. There are downsides to a brake, especially on a hunting rifle.
KGL
But if the brake is removable, you can use it for sighting in then pull it off for hunting...?
You should sight in the gun in 100% hunting configuration. Btw, I don't believe scope damage will occur due to a sled. How does it differ at all (as far as the scope is concerned) from a 24 lb unlimited benchrest gun vs a 9 lb rifle in a 15 lb sled? The scope is still mounted over top of 24 lbs. The SCOPE'S mass will cause force on the screws and mounts because they have to accelerate the scope to the same velocity as the action. If anything the scope should see LESS force with a sled as the acceleration of the action is reduced. But there is MORE force at the action to stock interface because you are essentially adding weight to the stock, not the action.
I have sighted my hunting rifles in using my sled and then tested point of impact without the sled... I found no real difference...
It only makes sense if you use a sled to compare any differences in poi and go from there.
And with a sled that weight is added to the stock... reducing recoil and reducing stress on the scope and mounts... I think Korth is way off on his theory about a sled increasing scope and mount stress.