Even after the the water temp needle gets to 90deg your oil still isn't up to temp -- Which is a good reason to get an oil temp gauge! But it's a good indication of when not to hammer your car if you can't tell oil temp.
Also I've been told from various sources that letting your turbo cool down before switching off is an old myth from when turbo cars used to run on old style oils... if you switched off your car when the turbo was hot the oil would "coke" leaving carbon deposits which could score your turbo and cause it a premature death, aparently with good semi-synth and fully synth oils these days this isn't a
problem... But I still err on the side of caution and let it cool down. If I've given the car any stick, I drive the last few miles off-boost and give it a min or two at idle outside the house.