Hi Paxboy
one of the limiting factors on a subsonic aircraft flying very high e.g. 777 is running up against compressibility effects. This can cause flutter and ultimately structural failure.
However, Concorde is obviously designed to completely ignore the high subsonic region and just kinda plough straight on into supersonic flow.
Don't know about stability and manouevrability in supersonic flight.
Enjoy!
(envy)