At the end of the day they are the 'professionals' and i use that word lighlty, and in the eyes of the law, they owe you the duty of care to provide you with the services with which you agreed upon. If things go wrong then they should be liable as it was a part of this service.

Being their profession to provide these high powered engines etc they should have sufficient knowledge in what the car should be able to handle and if they beleive the fuel system was not the right choice they should of said so. You have covered your self by questioning their choice in the china spec wastegate.

This is what i can remember from doing a semester of doing engineering law,and i would say you would have a pretty good case so dont just let in and throw in more cash to get it done, stand you grand and get these guys to get it right.