At 50 years old, Brendan looks as good as most men at that age. Maybe not movie star good--but he's gone through a lot of crap and suffered for it, and certainly doesn't have the funds to pay for personal trainers, plastic surgery, good hair transplants and trips to fat farms. Besides, Cliff looking like a mess is part of his character arc--and they probably made Brendan look a little worse off to play that up. He doesn't get a lot of scenes in the flesh, but the ones he does get are gold. And his voice acting makes this show the great work that it is.
Matt Bomer gets a lot of scenes as himself. As we saw in the latest episode. And I think the burned up Larry, without the bandages is also Bomer not his double. The acting that both doubles do is more than just stunt work--it's real acting (just without words)--so I'm not sure what we should call them.
"Danny Patrol" was a beautiful episode. I was just so happy with it. One quibble is that after some creative ways of showing Danny's messages, they fell back on conventional devices to do that. Maybe for budgetary reasons. I like it when the words show up in unexpected places.