I've shot a few custom slug guns one made by Tar-Hunt and they shot realy well but for the money I don't feel that they are worth it.
The thing thats gonna limit your speed in a slug gun the case , shotguns are limited to about 12,000psi
I've done some testing with shotguns by cutting the barrel back to see where the velocity stars to drop off and that generaly around 16" so by getting a 20" barreled shot gun the only thing you would be loosing would be site radius.
I have a couple tactical shotguns that wil shoot the old Foster type slugs into centermass all day at 100yds off hand and they are smooth bore guns.
With a properly assembled Rem 870 with a fully rifled barrel and the new sabot ammo out their I don't see 175-200yds being out of site. I bult a turkey gun for a close friend of the family on the Rem 870 , using a 21" rifle sited Rem barrel that was backbored , forcing cone lengthened and the Rhino choke lapped to the bore , the trigger worked and threaded pin instead of the friction type pins that hold it together it shoot realy well , better than any Benilli of Bereta I've shot.
But check the regs , if you can hunt with a handgun get yourself an Encore with a 308 barrel and you'll be good to go out past 400yds.