Wow, I can't believe the TT doesn't have an oil temp gauge, that's very surprising. As a rule of thumb, the oil needs to exceed a certain temp before you get on boost, as the the oil is very viscous at temparature and may have trouble moving through the turbo feed lines.

As far as turning off, you do not want to turn off the car when the turbos are extrememly hot, as the oil can coke inside the oil lines. This is no longer a concern with synthetic oil, but it is still good practice.

I always let my oil reach 175 before getting into boost, and I have a turbo timer set for 40 seconds, but it adds time based on my driving, and I flip it to a 1:20 if I've just driven hard...