I have the same problem, when the car gets really hot and your stuck in traffic in idle and then go to pull forward it bellows out blue smoke. that has collected in the exhaust then clears.
When I purchased the car it only did it every now and again and it was advertised as valve stem seals, but without having the head apart I cant see how he could say that was deffinitely what it was.
but your right could be turbo, piston rings, valve stem seals, but deffinitely not valve rocker cover gasket as that isnt a gasket stopping oil getting into the combustion champer thats just keeping oil in the valve train.
So Ive taken a gamble (being the cheapest option) and purchased a recon turbo unit to fit to hopefully cure the problem.
If this doesnt fix it then I will be looking at a second hand low mileage engine to buy