One thing you could try is just not having distance textures for some of the small or stubble states. Just for the big green and maybe harvest states. You just leave that distanceMap="" part out of the <foliageState> sections. The reason I suggest that is because, with just the 31 max vanilla crops at 15 available growth states per crop (again vanilla). You would end up with 465 textures if they were all defined.
Might be something where you have to do the balancing act. More crops and less distance textures or less crops and more distance textures. Again, though, I'll ask around and see what I can find out.