We've kept both Siganus vulpinus and Siganus unimaculatus (We like to keep one in our frag tank) and neither have ever put a dent in bubble algae, LOL. They do a wonderful job with other nuisance algae though. They are also the only fish we've ever had that would actually eat bryopsis.
Our Desjardini tang will eat bubble algae if you pop it off the rock for him, LOL.
Thats probably the same experience I would have with them, especially since I feed too much LOL. I really don't want to add anymore tangs, especialy one that gets as large as a Sailfin. My 5' tank just wasn't meant for them.