Sorry but you're mis-informed by those who sell those timers and want to sell you one.
I own an old Volvo turbo car (oil cooled ONLY but now run synthetic) and repair Volvos daily. My car's now 18 yrs old with its original turbo, rebuilt once at 165K because the original owner didn't change oil very often and its exhaust housing had a crack from heat. Turbo rebuilt 8 years, 80K miles ago by me. I now have a used water cooled turbo setup for it too but havent swapped it yet.
Newer water cooled turbo cars simply aren't failing, even at high mileage with less than perfect maintenance and no one idles the cars before shutting the engines off. I don't, won't do that with my car either.
The only time when a person should allow the turbo to cool is after racing, where the turbo's been spinning very fast. During normal driving, even if driving hard on the freeway, by the time one gets to a destination (gas station, etc), the engine speed has been lowered and the turbo has cooled off enough to not cause problems.