The change to HP rated at the propshaft instead of the powerhead is significant! I’ve had a lot of motors on my light weight 14′ aluminum boat, and the 9.9 Mercs from the mid 1980s on up (propshaft rated) will run circles around the 9.8s (powerhead rated) they replaced.
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Note that in the 1950s, some things were again different. I have a 1957 Johnson ad that claims all their motors that year were rated at the propshaft, and the way they perform, I agree. In the early 1950s, the 10 HP Mercurys were known to put out a lot more than 10, and the 10 Hurricanes (KG7 and KH7 models) were rated in the advertising at "10+", and put out more like 16 HP or so depending on how many RPMs you could get them to rev.
Dave
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