Correct me if I'm wrong, but doesn't the GTI have a pump that continues to cool the turbo after the car has been turned off? The reason you let the car idle is becausethe heat f the turbo can cook the oil in the lines surrounding the turbocharger, so you have to keep the oil moving while the turbo cools down. The pump should solve this issue.
Also, "high sustained speeds" isnt an issue. The turbo only realy spools and builds heat while under load, which happes during hard acceleration. Typically, an engine will run at roughly 20lbs of vacuum. Even a turbocharged vehicle, at a nice steady cruise, regardless of the speed, will remain roughly around 15-20 vacuum. So while sustained highway speeds aren't an issue, I would alteast recomend that you drive your car like a sane human being for the last few minutes or so.