Its not only the turbo boost that's increased, I thought these cheap boxes increase the fuel rail pressure too?
Many people will most likely tell you they've had a box for x thousand miles and never had a problem. But its a bit of a gamble with your warranty and only you can decide if you want to take the risk.
Its funny how we hear of many examples where no-one has any problems - but hardly hear of those that do have a problem.
I've had problems with tuning boxes in the past, but not on BMW's though.