Curious why you think the stock Jag rear inboard brakes NEED up-grading? Being inboard the biggest problem your likley to encounter during RACING conditions is extreme heat. Larger brakes may help some, but will only delay the inevitable if your racing. The stock brakes, might change pads only, are really quite adequate for all but the most punishing environments.
I'd be more concered with how to get some air scoops rigged up to keep them from melting the differential seals, overheating the rear end
oil and such. Changing pads and bleeding the rear (which is critical if your racing) is a real nightmare with inboards. On the ERA the panel behind the seats can be removed to gain access, if not for that even bleeding them is dam near impossible!