It is common knowledge that the 525EFI headers have an extremely high failure rate. I would hope that someone with 30 years of marine hi perf experience would know this.

As far as Mercury High Performance, not knowing what they are doing.........They do, but they have still made mistakes. A couple examples being the reversion problems caused by Gil exhaust on early HP500's and valve spring issues with HP500's. The solution was to change both of those on later models.

Griif.. good info,, but I am concidering 500's. What years to stay away from? and what years are safe??

All the 500EFI's are fine, which is whats in most of the stuff you're looking at. The exhaust risers on the HP500 carbs with reversion issues are in the first pic. The newer ones have a taller rise at the elbow like second pic. Pretty much all of the carbed 500's need valve springs by 200hours.