More gears with shorter ratios are great for keeping high-revving motors in their powerbands. Diesels are low-revving motors, so they do not benefit.

The current 6-speed works perfectly with the 35d motor. The only reasons why BMW would change to the 8-speed (IF they change it) would be for economies of scale, and/or for higher highway fuel economy (if the top gears are taller/overdrive ratios).