27' O'Days Did Come With A Yanmar Diesel...
...I had one. 8 HP as I remember. She would not hit five knots under power but had a range (theoretical) of 500+ miles. I never even gave a thought to fuel during the season. So, yes, it was a bit underpowered and she did shake, but she was still a very nice engine. I also had a Columbia 28 before that which had an Atomic 4. No question that it had (too much) power and one had to take a bit more caution with gasoline. However, it was quiet and had little vibration, so it was a sweet engine in that application except for the fuel consumption (or more properly, capacity).
If I had a boat with an A4, I would rebuild it if possible and do all the available upgrades unless I had a cruising agenda or my use was materially changing and for some reason another boat wasn't in the cards.