Nope, they're good for looks (opinions also vary on this) but for the price, you won't see much increase if at all in stopping distance with larger rears. I looked into that Baer kit alot when I was shopping for brakes but in the end decided to stick with the stock calipers/rotor size and just put new pads and rotors on it and paint the calipers to match the new fronts.
But yeah, you won't believe how ****ty the GT brakes are until you slap-on the Cobra stuff. The difference is night and day, especially on the race course. The MM LCA's did a noticable improvment to the way the car handles the twisties, but the brakes take the cake in this decision.