Your numbers are normal for the prop change you mentioned. Dropping pitch gives better torque (for getting water toys up and going, pushing heavier boats) but reduces speed and increases RPMs usually. OMC went with 4500-5500 RPMs WOT range for many years with the rated HP at 5000. They had big blocks to get the torque which made for great pushing ability for the engine size and ran at lower rpms turning large diameter/shallow pitched props. Back in the day (1950s era) a 35 hp engine was a very effective power plant for pulling a skier on a pair of skis and would run a 16', wide beam, bay fishing boat loaded down with gear and folks at a good clip. I learned to water ski behind an Evinrude 18 hp on a 14' wooden boat......wasn't stellar performance but it got the job done and was all that I could afford at the time....lucky to have that.
Somewhere around the late '50s they introduced a streamlined lower unit identified by a pointed nose cone on the gearbox vs the old style blunt nose cone. I think the Evinrude "Lark" carried it. I recall it being smaller in diameter and much sleeker. No idea as to the gear ration difference, if any, in the two lower units. Also in the 1958-9 timeline the "Fat 50" OMCs first V4 was produced and it had a larger diameter lower unit turning a 13" prop vs 10" for the lower hp engines of the day. I would imagine that the different hub diameters you mention would have to do with props that fit the different styles.....all vague memory conjecture!