Quick look would show that the diff of air properties (all at 100C ) between 0%RH and 100%RH, would have the heat transfer over the fins be about 25% more effective at 100% than 0%. So, all else equal, moist air is a better HT fluid than dry air. I should add that the effect is greater the higher the bulk temperature of the air, with little difference at 0 C, where of course there is a whole lot less water vapor per cubic ft of air than at 100 C. This would be absolute best possible case, and assumes the air is either all dry throughout or all wet throughout. Clearly of you take 80 degree air at 100% RH and run it through your fan and cylinders, it will emerge hotter, and hence at a lower RH. The 25% augmentation number quoted assumes RH is 100% throughout, which it could never really be, absent any water addition -- which there is not. In the real world, and I'm just estimating here, the relatively humid ambient air will work better than the relatively drier ambient air, but likely the net effect would be less than 10%. what really counts for air cooled engines, as you all know, is the ambient air temperature (duh!!) and the density, i.e., altitude, with hot, high air being the worst.
Yeah, TMI . . .