I do remember a few month back that Dinan had a 750 in the shop and was working on a tune. They had an X6 in their shop for like 7+ month and had worked out a turbo tune and were continuing the development on the 4.4l V8TT engine on the 750 since they basically have the same engine (X6 50i and 750). Not sure if Dinan has actually released anything for the 750 though...
Personally I wouldn't mess around with the car unless the warranty was expired. Considering how complicated these engines are these days, if something were to happen to the engine due to the tune, good luck getting it taken care of by BMW... Just my 2 cents worth
