Interesting. I've researched this quite a bit as I just bought an 2002 F225 (port engine) from a bigger Grady. It had the exhaust update done. You can tell if you look under the pan and you see some green metal vs the normal Yamaha Grey. Yamaha's update parts are a different color that stock. For whatever reason it doesn't affect many motors up in the Pacific Northwest. I called 3 yamaha shops in the area. 2 had never seen a problem motor, 1 had seen 1. Not sure why. I bought my motor from Canada and it had the update for $5000 U.S. 1200 hours. Looks almost new. No corrosion, hard to believe on the west coast of Vancouver island, almost all salt. The boat is from Alberta though. Do people use 28' Grady's in Alberta? I have no idea.
POTENTIALLY affected motors all Yamaha v6's until 2006. 200's, 225, 250s (Not sure when they started making 300s). This is a dry exhaust passage, flushing won't help. It's more mysterious than that on which motors are affected. Like Above posts say. Only 1 engine had an issue out of twins.
You can get the update kits at:
https://www.simyamaha.com/searchresults.asp?Search=225+exhaust&Submit=
$650-$740 depending on year.