RB, as far as I know, I've not seen any kind of standard test for turbos. IMO, if there was/is, I'd think that schaeffers would post it on their td's as they post ALL of their test data. Look up the tech data sheets on those "turbo approved" oils and see if they show any technical tests to show turbo approved. I suspect it is nothing more than a marketing ploy.
The best way to determine if an oil is going to hold up for a turbo is the ht/hs. If it meets the euro specs a3 etccc, then it will work in a turbo no problem. I'd disregard any standard api spec for qualifying the oil's ability to stand up for turbo's.