Is performance really not a large issue for wheelchairs? I would think the elderly would not have the strongest arms, and younger, powerful riders would in many cases prefer speed over comfort. Yes, it might be useful in rough terrain, but that is precisely the terrain where the loss of power due to inefficiencies in the wheel might mean the difference between getting over that bump or not getting over it. I think there is little room between this and just mounting (much) wider tires and using less tire pressure.