Lots of factors here...
Remember today's corn planted at mid 30,ooos vs 18000 in 1960 will yield more- so that is more load.
ground speeds have increased. Wagons are bigger. Choppers with kernel processors pull harder.
When I was a kid, Dad could run the Fox 2 row with the 4020, but the 5020 handled it a lot better.
90-100 hp would be bare bones minimum in today's world.
Today, we use the 4850 (215 or so)on a 3 row gehl, and in really big corn 3 mph is it, epecially if the wagon's gettin' full on an uphill grade.