VNT is used on jet engines across many different applications, so I can't see the temps of a turbo causing too many problems if its built correctly

Whilst I don't know a lot about the applcation of VNT on turbochargers etc, I can't see the general principles of the systems operations being massivly different, and on aircraft engines the VIGV and VNT (on the compressor and turbine respectivly) are used mainly at lower engine speeds, to try and smooth out the power delivery and stop surging due to bad airflow through the engine as it accelerates.

As far as I can see, the VNT on a car turbo would be used for the lower RPM range, to give extra responsiveness on a larger turbo, so top end performance mapping shouldn't be a problem as the VNT on a turbo should be in its fully open position as soon as the turbo is on full boost, and the lower end operation is controlled by the ratio of manifold pressure to the exhaust pressure, so to control the fuel map etc a decent closed loop lambda system should control it quite happily.