Guess my experience has been about the opposite. I've always had better accuracy from the 12 bores. Would not recommend foster slugs for game. Go the Brenneke route, especially for stuff bigger than deer. Foster slugs, i.e. Remington, Federal, Winchester "rifled" slugs, are poor penetrators, even more so in the hollowpointed versions. I've seen too many fail to penetrate even a deer too think about them on bigger stuff. When you can't take a quartering away shot on a deer without the slug stopping in the paunch on the way to the far shoulder, you know the set up isn't good.
A very good hunting chum of over 30 years has always been a 20 bore user. He has finally seen the light, having had inadequate penetration and killing ability on about a 1/2 dozen whitetail over those years. If you have the chance to see several dozen deer shot with both gauges you will see a noticable difference in the affects.
A good smoothbore slug gun, shooting ammo it likes (they're just like .22 rimfires, you have to find what works best) should be a decent 125 yard big game gun. With a rifled barrel and decent sabot ammo, you might be able to stretch it to 175 if you _know_ the drop for dead certain. Even sabots drop like rocks past 125 or so.
My slugger like the Brenneke MP, but try the Vitt slug too. I believe Precision Reloading has them. THeir a handload only item, but I'm thinking it may be the way to go with more load development.