Compare the load/weight ratio of different luggage systems, and you will find that seatpost mounted racks with small panniers are fairly inefficient. They work OK for small loads, but a Carradice saddlebag is far more effective. A buddy of mine rode his racebike halfway across France with a 20l saddlebag and a barbag.
I did meet a guy in Ireland touring on a Bianchi Pista track bike, so anything is possible.