I had a 20 gauge rifled barrel for my Encore. It's a really nice rig as a long gun. Light, fast handling, well balanced. I never even considered trying it as a pistol.
However, I've always thought 20 gauge and .308 were comparable recoil levels in long guns, and I have shot my Encore in a pistol configuration with a .308 barrel. Recoil is stout but bearable.
However, let me point out that 20 gauge ballistics (using a sabot slug from a rifled barrel) are very comparable with 44 magnum ballistics from a long barreled single shot pistol (Encore/Contender) . Unless you've already got the 20 gauge barrel, going with the 44 magnum makes more sense, especially considering the cost of feeding the 20 gauge sabot slugs.
Michael Courtney