TDI tuning are offering a chip for the 30D that will take it above the 35D, 323PS & 700Nm. I understand how these work, cylinder temps had to be reduced to get NOx for euro 5 so there is plenty of spare in the engine. It should also improve efficency so the fuel improvement should be right but are they traceable at a service once they are removed?
http://www.tdi-tuning.co.uk/car_and_..._kw_P2573.html
I had a chip on an Astra I had years ago, 120,000 miles in 3 years and no mechanical problems, always removed for service and no issues, but has anyone chipped a F25 X3 yet?