I would not give up with it yet. The Canberra on both sides of the Atlantic was designed exceed 400kts with no trouble, so there is definitely something amiss here. I struggle to get mine up over 500kts in level flight, but it is possible with a stripped down payload and fuel. This is not accurate admittedly, as it was capable of getting up there and beyond relatively quickly from I understand. In the end I had to settle for a compromise in my FDE to retain good handling at low to mid range altitudes rather than overload it with the thrust necessary to achieve these speeds and better high alt performance.
Rich, if Marc put in the Airfile from my B-57A and it was still shaking, then its a head-scratcher, as I use FF too and have never had any problems with shaking even over 500kts. And no, the airfile was worked out between me and some of the beta testers so don't worry that this is anything on you!