I've read in a few places that it's possible to get plenty of horsepower of the the twin turbo diesel, I've also read that tuning more than an extra 50 bhp has serious implications for the rest of the drive train.

Quite a few companies in the UK offer tuning but I haven't seen any that offer a warranty offer more than 50 bhp.

This company has an excellent reputation and really know there stuff on BMW's. The owner is a regular contributor on the best BMW forum in the UK mtorque.co.uk

Thorney Motorsport Bluefin Remapping (BMW)