I'm really worried about some of the expectations you guys have for headliners and sub headliners. I seriously won't be surprised if someone says they think U2 is headlining and No Age is going to sub headline. Why can't you just choose to set your expectations really high or really low, not a mix of both!
Actually, considering when I went to Pitchfork festival (LOLZ PITCHFORK, GOD, FUCK ME), Robyn played before both Broken Social Scene and Modest Mouse, and remember Broken Social Scene played 4th to last, at '10 Sasquatch, however I think that was decided for other reasons than how big each one was. Dunno though. Maybe at Sasquatch she'd be a bigger deal though.
Short answer: I think she isn't QUITE big enough this side of the pond to subheadline.