As far as my knowledge on the matter, watercooling is generally to keep the oil in the turbo from passing into the higher temps that cause coking. Coked up oil inside the turbo will inevitably lead to improper oil flow and eventual starvation, causing failure. Of course you could argue that oils have come a long way and thus this might not be necessary. I imagine this sort of extra safety net is for people who don't change their oil on time, which we all know is very important for turbocharger longevity. As far as hard evidence? I don't have any.