Actually, thinking about this, I would think that 300 pounds and 5'10 would be more than fine on most folders, but 280 pounds and 6'3" would be ideally left to the Bike Friday or Moulton folks. At issue is the fact that the extra long seat post will have a good deal of stress on it at the point in which it goes into the frame. Of course, you could always go steel, which Brompton will do.

Remember that bikes designed for high load touring have a lot of extra metal on them. The Birdy frame is now 1Kg heavier than the earlier frame designed for commuting alone. Everyone with 8Kg Birdies on their forum have older frames.