Not sure they are a big improvement but there are these GM caliper replacements for the front.
SSBC Quick-Change Caliper Upgrade Kits A185-S - Free Shipping on Orders Over $99 at Summit Racing
I think there may be some Sierra brake kits to step up even further for an increase in rotor size.
The Jag suspension works fine in most driving - I would think that the more you beef up the front brakes the less the rear brakes will be doing. But tucked in under there they don't get much cooling and they are running fairly thin, solid rotors. There are some ways to modify the rear brakes by installing spacers in the Girling calipers to fit a wider, vented rotor, but from memory it's fairly expensive.
Basically, if you are really going to run the car hard on a long track, you are going to need big brakes and the ERA suspension is the most straightforward way to accomplish that - but at considerable cost. If you end up going that direction then you might as well really open up the pocket book and get a 6-pin conversion for the front as I think most if not all ERA rear suspension have 6-pin hubs.
Post some pictures of the car when you get a chance.