I've seen plenty of people touring on race bikes with seatpost racks. Some with larger panniers than mine. One problem for a friend of mine that used one was that the geometry of the frame (short chainstays) made the load sit rear of the rear axle, which made the front end very light. He wasn't ultra-light (we were camping and cooking) so it might not be a problem for you. You could aslo get a handlebar bag and put a few pounds in there, it'd help a little and also bar bags are nice for snacks and valuables. I keep all my valuables in my bar bag and take it with me when i'm off the bike.