FWIW, I think a 57-card speed deck would be preferable anyway, since with Speed you're not usually going to depend on soul-drought so the extra soul won't hurt nor will it add to the amount of cards you need to get through, and you get to use an extra dominant.
As for whether 56/57 is viable, I'd say it is, but it's not optimal. There are going to be games when the extra defense will come in handy and you'll get the advantage over other speed decks, but I'd say overall your chances are better the smaller your deck is.
