I've had a few different 27.2mm seatposts in this frame, and have had no problems with any of them slipping despite never tightening the clamp bolt all that much - the seatpost fit is very good.

The clamp is certainly all the way down, and you can see a light ring marked on the seat tube in the 2nd photo showing where the bottom of the clamp always lies. The clamp is not exactly low-profile, but even with a different clamp I don't think I would gain any more than 1mm, maybe 2mm, but that would not be enough, I need at least 5mm.