I let mine run until it cools before turning it off, luckily ive got a 3-4mile stretch of 30mph road to get to my house, gives it a nice 5-10mins to cool down! It makes little or no diff to the overall coolant temp ime and as gadget says the cores are different too so the s2 core won't be cooled by anything other than the oil thats lubing it.
If you are fussed about the hot water from the turbo heating up the engine, fit a turbo cooler rad...