When my STi started smoking like that, I was using Mobil 1, and the dealership couldn't diagnose it, but it turned out to be a blown ring land. I know you didn't want to self diagnose, and I see it the same way as you do, but when they kept giving it back telling me that it was "normal," I had to do something. They've strayed from actually servicing vehicles to only working on ones they know will turn a profit, and some of the diagnostics tests are labor intensive with few results on occasion. It took a couple hours to compression test it, but the dealer wouldn't do it, because they said the car was running "normally." Good luck with yours. I hope it's not that bad, but mine only had 19K on it blowing the ring lands. I know it overboosted too, because I cleared the ECU and the car almost immediately boosted to 22 PSI. That's not okay. Let us know what you or the dealership finds out.