Thanks for the detailed reply Brian.

I have asked Yanmar about the 3" exhaust. Our system is 50mm, say 2", and would be a LOT of work to change in our particular boat.

When I see 3" exhaust specified on all Yanmars up to about 125HP turbo, I question the need for 3" diameter on a 50 HP.
I asked the local dealer here, who sells about one engine per year. I also Emailed Yanmar Europe HQ asking whether they would support warranty with a 2" exhaust.
The Yanmar warranty simply says that Yanmar can refuse service if they consider that installation is inadequate.

It will be interesting to see if Yanmar HQ responds. It is very frustrating when companies insulate there strong technical resources behind salesmen that usually pass messages poorly, and sometimes add BS to help sell.
The access to tech resources at Beta appeals to me. I have sent their English HQ a couple of questions too, and have proposed a visit to the factory in September, when we will be taking a week in London with friends.

Did you winter in St Catherines Dock? We have bunked in a friends boat a couple of times there. Great place, but difficult for working on the boat.