[joyridin']

What style caliper does the rear brakes use? Jaguar had a couple different styles through the years. The early style with bolt-on pistons were real easy to modify. You just find a set of front piston pods and bolt them on. Later years, the calipers were 2 piece and single piece and it is a bit harder. Just adding larger calipers is going to make the situation worse though. Now the master isn't going to push enough fluid to correctly work the calipers. In reality, you would want a smaller piston diameter to initiate lock-up. Now it will take less fluid to activate the calipers and you can put an inline adjuster to adjust the bias.

This would probably be a bit more work than you are willing to put forward, but ditching the single master and going to dual masters will really help the situation. You can now adjust the bias any way you like.