We can get info and parts, but we don't work with them much. Merc outdrives are certainly the market leaders. On a stern drive, the cheapest way to get speed is to go with a Super Charger. We usually see about a 10 mph gain on most boats. Plus, there are now models available for the newer EFI models.
Stainless props can make HUGE differences on the right boat.
It depends on the boat. That should answer a lot of questions for you. That's four more than we've seen here in a while. Was it the same problems or just general failure. We have three OptiMax 150s rigged on a ski boat.