Sounds normal to me...around 15k miles. My rears always go twice as fast as my fronts on my 350. I dont know why the hell your fronts were going faster on your 350.

Anyone that drives a Z and gets more then 20k out of their rears drives very conservatively or has ZERO camber.

Ya I'm pretty sure once my springs and spacers are in next week the tires will wear a little quicker. Seeing how I know my camber will most likely be a little off.