Set flap 1+F at F200 in the 320 sim once when we had some
spare time left over.
For the hell of it set 390 in the window and allowed speed to
follow Vx increase as altitude increased while not allowing it
to exceed 1+F limit. Passing 330 we got some buffeting and
deduced it was Mach effect. We continued climbing when Vx
encroached on Vfe 1+F, still buffeting but more pronounced.
Didn't fall out of the sky and might've made 390 if the sim
time hadn't run out.
But its just a sim after all and probably doesn't have the necessary
algorithms to fully reproduce these effects correctly.