Well, look at it this way . . . would you not use a .45-70 on deer, assuming that the shots were under 100 yards? I know I sure as all getout would, if I had one. A 20 gauge sabot slug is ballisically identical to a .45-70 bullet . . . come to think of it, it's the same bullet, depending on whose manufacture of slugs you're using.

Based on that, I'd say that there's no way you should feel undergunned with a 20 gauge. I know someone who still uses the old Foster type slugs in his old Browning Auto-5 on deer. Inside of 50-75 yards, it's "Goodnight!"