I have never shot bird shot with a scope, same with buck. It seems silly to me to try and use a scope while swinging for birds.
I have shot a Remington 870 with a red dot, with slugs, and saw no improvement in accuracy, as opposed to using my bead sight.
If you want a really accurate slug gun, buy a rifled barrel for your shotgun. For bird shot and buck i think optics of any sort are unnecessary.