I have a 70' seed hawk drill. You're correct with the location theory of manifolds for folding, although a couple could be moved a bit, but may not gain much. I have mine on a Bourgault tank, with Bourgault secondary air kit. Takes big air, but can run 350 lbs. Bourgault fans respond different than seed hawk, and to do that our air is running 5700 - 6000 to move that amount of product. Outer 2 sections are prone to plugging due to length, but we are able to seed at 5 mph.
On the Case side of things, we ran a twin pump as well. Both fans off of second pump. Had Case turn pressure up to 3100 psi, factory is 28 - or 29 I think. Also we switched the 4 fan couplers to 3/4" replacing the 1/2" blocks on the tractor. It's an easy swap, and not that expensive. Might help.