I don't have any personal experience with the dry exhaust corrosion issue but when I bought my boat last October, which also has a 2004 Yamaha F225 on it, the dealer I bought it from who also happens to be a Grady White dealer, said they pulled the lower unit and inspected for the problem, of which they didn't find any issues. I trailered the boat home 3,009 miles from Maryland to just north of Seattle where I live, and the local GW dealer in Edmonds is called Jacobsen's Marine. I had them winterize the boat and drop the lower unit just to make sure the dealer I bought it from was being honest with me, which they were, and Jacobsen's said everything looked perfect. That being said, when I went to pick up the boat, I asked the tech about the problem and he said it's about $2k for the parts and labor, and if you can catch it early enough then you save the motor and just have to do the parts kit and labor like you've done. But... he said if you let it go for too long and it starts to spread into the other parts of the motor where the kit you replaced touches the other parts, then it can mean a new powerhead or even worse is having to get a completely new motor. He said that they've done dozens and dozens and dozens of these motors before, so I'm fairly confident that he knows what he's talking about. I've heard a rumor that oftentimes the issue starts around the 500-600 hour mark and if you can get past that without any issues, "typically" you won't have any problems. But obviously it's not guarantee. I'd contact your local GW dealer or Yamaha repair shop and see what they have to say about it but can't imagine that Yamaha would offer any sort of repairs, warranty or anything on an outboard that's almost 20 years old at this point. Good luck and let us know what the outcome is!