This is the first report of a cracked turbo housing we've had. Why did you rebuild the wiring harness and not get your dealership to call GM for assistance? 50K miles for spark plugs is probably about right given the "change interval" is 60K miles. If you lug the engine at all you'll erode the plug gap faster. We have had a number of ignition coil boots tear - it would be nice to be able to purchase these boots by themselves.