First off, jag calipers have mounting bolts in one plane, XD have them in two, so forget that.
Thers nothing fundamentally wrong with XD braking system, 4 piston calipers wouldnt improve the situation a great deal. Bigger discs would give more benefit, but bigger wheels would be needed to cover them.
Ive heard that Volvo calipers are easier to fit, but again why go to all the trouble when braded stainless hoses will most likely make the thing stop, with a better pedal feel.
The front end swap to a HOLDEN is not to ne mentioned and the work involved would be immense, and it would be worse than the ford, and to the transit owner it would be too narrow in any event.
No overall id be looking towards whats on the BA premium braking system, but the money would be more than the xd is worth.
Get rear discs for the xd, or, just by an xf with them and PS standard. then sort out the front end with bushes and the like. The biggest problem is the brake reaction struts joints, they compress under brakes and go to negative caster which makes the car unstable.
A7M