I worked for a dealer that sold Mariner/Mercury up until 1995. Then they went Yamaha. Before the change over, there was an overlap period where there were some new 25 Mariners in stock and some new 25 Yamaha's.
I spoke with the Mariner rep after hours and he specifically mentioned that the rating for the Yamaha was at the propshaft and is why they were faster, and less reliable (according to him...he's trying to sell motors)...where the Mariners were still rated at the flywheel.
After the last Mariner was sold, they never looked back.