Not sure the alloy was changed, the coating material definitely was changed. The combination of salt water, exhaust gasses and heat were breaking down the old coating. Once it got past the coating, the base metal was corroded very quickly. I only had small breaches of the coating but could see the pitting of the base metal and in some places corrosion. I was told by my dealer that Yamaha had a heat map (number of failures recorded by continental US map) and the larger percentage of failures were in places with warmer water. I never saw the map - just relaying what I was told.
I probably would have gotten a few more years out of mine, but because I had all the parts and the motors were apart, we did it. So far so good.