I don't think Seven is underrated over here. He's the third wheel in BSS, but that would be true of almost anyone.
As for moving on, both Dunne and Bate are full time WWE now, so are a handful of other UK signed guys (but not Seven). If that means they are moving to the US soon isn't known yet, but they have to eventually.
I don't think they'll go as a group. Dunne is a heel, Bate is a face, And both are great singles guys already. It's the end goal, so being singles off the bat makes sense, but I do think they should be used as a group initially. Especially on the main roster.
That being said, it comes down to what the roster looks like. If you've got other groups going then adding BSS to that would be great, but if there's no one for them to face then there's little point. Down the line you can always unite them if you want to.