Of course Yamaha looked into the problem but they never published results.
I know there are theories about fresh vs salt, cold versus warm weather etc but not of that explains how some folks with twins that were rigged at the same time, had issues on only one motor. That would lead me to conclude that the problem is a quality control issue and there may not be an easy way to determine which specific motors are more likely that others to experience the corrosion issues.
Yamaha's response was to special price the exhaust rebuild kit and the pricing was aggressive. The owner however was still on the hook for the parts and labor. As some have noted, corrosion issues have occurred on more that just the 225s.
The thermostat housing corrosion may or may not be directly related to the exhaust issue. Who knows, the root cause of the corrosion may be electrolytic in nature where some of the materials are more subject to electrolysis than other models, years, production runs. It's all guessing on my part but what I do know is that I would steer clear of at least 225s and 250s in the early to later 2000 model years.
Now, don't get me started on the first couple of generations of the 350