It does depend on the tractor...
Some of the Fords that I'm thinking of are 90% the same over a span of 25 HP and a couple are 99% similar over a 10 HP span... it's just that the lower power one is derated or the high idle is lower... so turning ti up is not a problem. On the other hand, some are substantially different. The 3930 and 4630 are nearly identical twins other than an overdrvive transmission ratio and smaller wheels on the 3930... and it's just rated at a lower speed with slightly less fuel delivery. It could very easily be turned by 10-15 HP at no detriment to it's design.
On the other hand... older models like the 3000 and 4000 are very different tractors with different displacement engines, etc and turning a 3000 up by that amount is probably asking for trouble.