I believe it's a maximum of 30% w/ Rabid Dogs. Also, you can have five (!) zombie dogs summoned with their damage buffed by 70% with Zombie Handler and Midnight Feast, for a total of 51% weapon damage per swing, although I'd probably pick Burning Dogs myself.
I think that Burning Dogs damage comes on top of normal bite damage, while Rabid Dogs change bite damage itself, but i may be wrong.
On top of 5 dogs there's a legendary item that makes pets attack 100% faster, and another one for 50% increased pet damage. Add 120% per hit fetish, possibly buffed gargantuan and we're set for a really strong build which doesn't really need too much defensive stuff.