How about touring with a saddle pack on a race bike? I have no comfort issues with my bike, and I'm not heavy myself, so my total load would be something like 160 lbs or perhaps a little less. My bike is aluminum with carbon fork and seatstays. The saddle is carbon, but if I were using a saddlepack, I would swap it for an aluminum one. My wheels have enough spokes to support more than 200 lbs of weight, so I'm not worried there.