Ross points out what I alluded to. Modern radiators are very good while aircooled cylinders and heads have serious limitations.
The big issue with making a finned head cool is that while the root end of each fin is at head temperature, the fin temperature gets lower and lower as you go further out the fin. Much of its area does not have the high temp differences implied by simply looking at CHT. Well, unless the fins are simply too small, and then it is game over anyway.
In a modern radiator, the tubes are small and virtually the same temperature everywhere. The cooling fins between tubes are short and heated by tubes at both ends, so they tend to be pretty hot everywhere too, so while the temp difference between fins and air looks more modest, they are pretty good. Add in modern design and manufacture plus good ducting and they can be substantially more efficient than aircooling. Or if you use old style radiators and oblique diffusers, they can be a lot worse.
None of this changes the fact that a water moderated system with cooling fins will require roughly three times the fin area of a conventional air cooled system because the delta T between base of the fin and cooling air is about a third of what it would be conventionally. This is assuming you can actually fit the larger fins in and also assuming that you do not get to the point of the system becoming an insulator instead of a cooler, which also can happen...
Billski