Here is my oil cooler, its behind the upper front grill, so its nowhere near my intercooler (ignore the red circle at the back, i took this pic for someone else wanting to know where the fuel vapour valve is), not sure how tall your intercooler is though.
Its not going to be as efficient if you put it behind the rad or intercooler because the air will already be warmed up a little after passing the rad or intercooler. Im looking into an oil cooer thermostat myself, as you say, theres no worry of overcooling the oil if there is a stat fitted, so bigger is better. Im not sure if there is an equation as to oil cooler size and design to get to the exact temps you need, there are far too many variables, core design, oil temp going into the cooler, position of cooler, and outside air temp.
Looking at your cooling system drawing, its possible you may have too much water bypassing the rad. The turbo will be dramatically heating the water up, and its going straight back into your engine via the oil cooler, theres hot water going via the heater straight back into the engine, and theres a feed that carries the water past the thermostat that comes straight from the head back to your water pump. Thats normal, but what size are the pipes carrying it? If they are quite big, then it might be worth putting a restrictor in the pipe before the oil cooler to stop so much water going through it. The water pump in the engine will pull the same amount of water into the block, but will pull more through the rad. Also are you missing a pipe from your header tank, there should be at least 1 more pipe like a small breather going to the head?
My set up has 2 small diameter popes, minimum is about 15mm inside diameter going from the head, one straight around to the thermostat so it gets an accurate idea of water temp comming straight from the engine to prevent over cooling. Then the other pipe goes from the head, to the turbo, from the turbo to in car heater (so the heater gets hot almost instantly), then from the heater back to the water inlet on the block. So the only hot water feeds the engine is getting is 2 15mm tubes. The main feed from the rad is about 40mm inside diameter, and theres just a small 15mm feed from the header, which isnt too hot anyway. When the water pump sucks water in, its pulling more from the rad then anywhere else.
Another option for you is to blank off the "T" that goes from your HCV to the header tank pipe, and re route it. Go from the head, into the turbo as you have done, then out the turbo into the HCV, then from the HCV to the pipe that goes into your oil cooler. That will potentially have better cooling on your oil when you have the heater on because the heat exchanger will release some heat into the car.
Just a few options there that you might be able to pick something from!
