For me, Anthony Warlow is the best Sweeney I've ever seen - both in terms of acting and singing. As a long-time Warlow fan, I'd love it if he had a chance to play the lead in a major Broadway show. (Groban sings beautifully, but he lacks the necessary intensity to portray a man obsessed with death and revenge.) And Imelda Staunton is the best Mrs Lovett I've seen. She's the only Mrs L where I wanted to see more of the character rather than less. (And her accent, naturally, is on point, unlike Ashford's.) But neither of these performers have the star power that is needed to make the Broadway production viable, though Warlow has played Broadway a couple of times in supporting roles, and Staunton is well known to general audiences due to her TV and film work. Bringing in foreigners is also more troublesome than casting Broadway shows with Americans.
Having seen Lea Salonga perform "Worst Pies in London" and "A Little Priest" recently at the Sondheim's Old Friends concert (the latter opposite Jeremy Secomb), I'd love to see Salonga play the role in a major production. And she's better known than Staunton in the United States, thanks to Miss Saigon, Les Miz and being a Disney princess. I think she'd be a fine choices for Mrs L, and being a bigger name than either Warlow or Staunton there'd be fewer issues getting her past Equity, I'd venture.