I heard they have 10.5 available for the m series now, but 9" stockers work well and unless you match the drivers on the front the track has still got to bend around them anyway , so changing the back wheels without doing anything to the front is just going to add to the "wow" factor a little.
It's not recommended but......You can get away with a 8.8" wheel without having to buy a drop axle. Make sure it's got rubber on the wheel though... lots of pressure on the track.