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Well, I'm no expert but...
Isn't there a valve that directs oil to the cooler only AFTER the oil has warmed up? IOW, the oil cooler doesn't see oil until it's hot so your turbo will be oil starved until the valve opens.
Also, from my past experience with an old T04 turbo, the oil feed and return is much smaller than the oil feed lines to the cooler. I think the turbo would be a HUGE flow restriction whether it's placed before or after the cooler.
And lastly, the oil that comes out of a turbo is generally pretty well worked over and can be rather aerated. The reason it should be drained back to the sump is so that it stabilizes before going back through the engine/turbo again.
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