I've seen literally hundreds of turbine housings cracked around the wastegate. In fact, when we had one come through the shop that WASN'T cracked, it was rare. In the cases that I would see the same engine a few years later for another overhaul, I never observed the cracks getting much worse until years later. Eventually the gasses will erode the metal around the crack, but that will take years and years, and 4 or 5 rebuilds later, and everything else will be worn out 3 times by then. This observation is based on my experience with marine and industrial diesel engines, and the housings in question were typically run red hot, 24/7, at full load for around 10,000 hours only shutting down every few days for an oil change. Then overhauled whether they needed it or not. I wouldn't worry too much about it, I don't believe it would effect boost whatsoever, unless it was big enough to stick a pencil through.