John, I got your e-mail, but considering the impact of your suggestion, it's better to reply here in a post. Others need to hear this too.
Use this on-line calculator to see the difference in what you suggest?
www.discounttire.com/dtcs/infoTireMath.dos
I'm presently using 245/45 on the front, and 255/45 on the rear, but in the Pirelli "P-Zero". P-Zero
Directionale tread pattern/compound on the front, and P-Zero
Assymetrico tread pattern/compound on the rear.
I'm very pleased with the overall balance in lateral adhesion, side wall integerity, and wear. My on-line calculator says that my tire diameter difference is .36", therefore relatively flat. I like that, my MM is only 4.5" off the ground at the A pillar. Many of y'all may not like this "slammed" look, but it serves to provide the best possible weight transfer on a hard launch too. If you prefer to retain the OEM rake, you can play with the air suspension setting in the rear to "buff it up" a bit.
John, your suggestion of 245/40 front and 255/45 rear seems very close, size-wise, but it's a yardstick away. Your size combination will result in a 1.32" difference between front and rear, and this difference suggests that the on-board ABS/TC systems will detect slip (or, skid) and react to that.
The difference should be 1.10" or less, for a compatable tire size.
Hope I have helped, keep us posted on what you discover?