It sounds like an 11 inch wide wheel will work on the rear with 305 width tires. You may need a spacer to push the wheel out a bit so that all of the extra width isn't negetively impacting the internal clearance.
Ordered: 2016 Silverstone F10 M5
Current: 2013 Monte Carlo Blue F10 M5 (ED on 9/1/12), 2014 Anvil WW Jeep Wrangler
Previous: 2009 Carbon Black M5, 2006 Silverstone M5, 2003 Sterling Grey M5, 2000 Anthracite 540i6, 1998 Jeep Grand Cherokee 5.9Ltd, 1990 Eagle Talon TSi AWD, 1978 Mazda RX-7