I am aware there are differences between fans, but from the test I've seen, they are not that big, to be really a crucial factor here. The fact that nobody tested 180mm fans properly doesn't mean they don't exist, or at least are not comparable to decent 120mm ones. I really don't see how larger fan has to be so much worse than several smaller fans equivalent, even without a shroud.
As for density, there is plenty of examples of different size radiators with exact same FPI.

You can Scale a RX120 -> 480... however YOU CAN NOT SCALE a RX120 -> 140 -> 180 -> 200mm
If you compare rads with same fpi, similar thinkess and similar effective fan parameters, you can easily compare it.