I really think that depends on your choke. With an Extra Full, you can stretch that to about 45 yds. But it is just not gonna knock a deer down like a slug.
10-20 yds? That is some thick vegetation! I would agree, however, that a load of buckshot at 15-20 yds packs quite a punch - assuming you aren't losing pellets to trees and limbs.
I haven't hunted deer with a shotgun in at least 15 years, but that is primarily because I don't have to. The shotgun is pretty much restricted to waterfowl and clays.
I've pretty much given up on clays.
No matter how I cook them they always taste like mud.