8-speed is for sport automatic. M3 is still being tested with 7-speed M-DCT. 8-speed is going to be trend given that Lexus LS460/600 and IS-F are using it. 8-speed also boosts fuel economy 6% more than 6-speed in similar application. The new ZF transmission has been designed with hybrid and all-wheel-drive in mind.
http://www.thecarconnection.com/Auto...73.A12292.html
It won't be difficult for BMW to apply 8-speed M-DCT in future model, one clutch controls odd gears 1,3,5,7,R and other clutch controls even gears 2,4,6,8, looks fairly distributed from mechanical stand point.