This is the problem with windmill farms, they take up a lot of space and the further away you get from the central windmill, the more axles you need to connect it to the central gear assembly, which results in diminishing returns and an eventual net loss of power once it gets large enough.
your saying the net loss is higher with a higher number of windmills, but that doesnt really make mathsense
if you got a line of mills {on x axis}, one next to the other, conected on their z-1 like this
G--G--G--G--G
each windmills gain beyond the first is deduced by the value of --G (7) so you still get a minimum 13 gain of each next mill (more with some maps)
now you can still make more {on y axis} with the same --G conection, still only costing 7 per next mill
considering the initial power cost of the drivetrain needed to make a windfarm highup, it only gets more efficent by having more windmills, not less efficient
windmills do take up a lot of space and wood, but are easy to make, and fairly failsafe
space can be saved if you build em high up, wood cant be saved, but theres usually enough wood anyway
anyways tastes differ, some prefer wind, some prefer water, some do both
its the beauty of choice and there is no 'one' correct solution
also, as allready stated, windmills are best when mounted directly ontop of a single pump (or a dual pump mister - but thats somthing i need to post in its own topic)