Andrew wyatt,
"would putting a clamp on it make it shoot better with slugs like the 870s, or does it already gain that benefit from the solid interface with the magazine tube near the muzzle?"
Best answer is to just go shoot it & see for yourself.
Sounds trite, but nothing like actually shooting to see what it does.
There's nothing anybody can say to do "this, or that" that'll work better for you than to just go shoot the thing & tweak it to what you want.
The whole ideal behind "patterning" your shotgun.