I have, with my son, compared the Remington 870 with a synthetic stock versus an 870 with a Knoxx system. Both systems are Remington Express 3" receivers with "plastic" trigger assemblies.
With dove loads, no problem with the synthetic so the Knoxx is not really needed...Shoots like a 12 ga shot gun. Solid and a no miss with dove load at 50 yards. (I have no experience with dove hunting, this is range experience!)
I am ignorant!
However...
With the slug load...ever felt a steel rod being driven into your shoulder, of course with a rubber pad to disperse the steel rod force?
That is what we felt from the synthetic stock.
We stopped shooting the slugs on the synthetic stock and (I) have since purchased another Knoxx for the weapon.
Using the Knoxx with slugs felt like using a synthetic with a dove load...O.K. not quite, a bit "softer" than a synthetic stock, and our shoulders were just fine after, 50+ rounds between us.
So, 'nough said...
If you are going HD get a gun which will accommodate a Knoxx.
Fire Fine, Fire Straight.
or
Don't waste the Fire!