Nope, If bigger wheels mean bigger brakes, I'm all for it...........especially on high horsepower, 5000lb+ cars and SUV's! I wouldn't mind both on my Jeep in all honest, a little bit bigger rim means a little less sidewall to flex during cornering and should allow for better cornering ability on top of the better stopping and heat dissipation the bigger brakes would allow for. I wouldn't go any larger than the 17's like your Mustang has. I've seen XJ's with 20's and they don't look even remotely right, 18's look ok, but that big might be kinda pointless on it.