I'm suprised you're finding that sabot slugs are shooting best for you.
Generally speaking, they are not designed for smooth bore barrels--they are designed for RIFLED barrels. Without rifling, it's very difficult to get a sabot slug to stabilize...my experience has them tumbling or flying very erratically.
However, they are typically lighter and faster (and of better design) so they can be great for hunting--you can take advantage of larger bullet design. A rifled barrel and sabots turns a shotgun into one heck of a powerful LARGE bore rifle.
That being said, my favorite slugs (and first choices if I ever hunted with a shotgun) would be dixie slugs out of my 20" smoothbore...next would be the Brenneke Black Magic. The latter is spec'd for 3" groups at 100yds--and I've come close to that with the few boxes I've tried. Both are designed to be TOUGH and are harder than most traditional foster slugs--which makes 'em great for larger game.
That being said, I find myself buying a lot of Winchester 12ga 2 3/4" rifled HP slugs from walmart since they're only $3.17 per box and do reasonably well from my 870...I mostly plink with them, but can't see why they wouldn't work well on deer with the 75yd range you've given.
You might consider a shorter barrel.
Most slugs optimize in 18-20" of barrel. Beyond that you're actually slowing things down (friction) and keeping the slug in the barre longer, which can decrease accuracy (and you take the recoil).
If you truly want to take advantage of your sabot slugs, but a rifled barrel. They can be had for ~$110 NEW if you shop around, and will at least double the effective range of those expensive slugs you're wasting through your smoothbore.
