The thing that concerns me about the mods to fix Jag brake bias is that they all seem to revolve around reducing the performance of the front calipers so the rears can keep up. Either a proportioning valve in the front or smaller pistons in the front calipers. A better solution in my opinion would be to put a better caliper on the rear to improve it's performance so it'll keep up with the front. The main problem I see with this is the hand brake is part of the Jag rear caliper design.
Fortunately there are alternatives now. Hi Spec are making a caliper with a built in seperate hand brake mechanism. They also supply a mechanical handbrake caliper if you want to use an AP or other caliper for the rear that has no hand brake setup.
Hi Spec handbrake caliper
Just another option.
Cheers