Seatpost angles are fairly constant so the seatpost clamp racks almost always end up horizontal.
Some have small side bars to prevent panniers swinging into the wheels. They can only take smaller weights and smaller panniers. If the pannier has a sound mounting system and good stiffening it wont touch the wheel.
Other racks to consider are the ultralight but very strong Tubus Fly which is very narrow (designed for fast touring so more aerodynamic). Old Man Mountain may make some which fit. Montague claim that std racks fit the bike although you may need their special stays to the seat-tubes.