The rear rotors are thinner to begin with than the front rotor, so they'll have a lower "minimum width" spec no matter what.

Factory fronts discs vary in size depending on model according to one website. My 35d has 348x30 fronts and 320x20 rears. Your '08 likely has 332x30 front and 320x20 rears.

__________________
Scott
On Order: 2015 F15 X5 35i
Space Gray with Mocha Nappa Interior Design Pkg and fineline Oak trim
ZXD | ZCW | ZPP | 3AG | 456 | 4UB | 4U1 | 5DL | 688
To be Traded: 2010 E70 X5 35d
Alpine White on Black with Dark Bamboo trim
ZAP | ZCW | ZPP | ZPS | ZRC | ZTP | 322 | 328 | 330 | 386 | 4AZ | 4UB | 655 | 6FL | 6NF