Happy new year Mark and thanks for the update. Yes with sand filters they only present a small surface area ( pi x r2 of the filter size) as the depth of the bed isn't considered for some reason so I would have expected the pressure to be lower with the large surface area of a pleated cartridge or DE grids. You say the headloss is about the same regardless of filter size, therefore they are very poor performers hydraulically speaking which doesn't surprise me as the pool industry designers are pretty lazy/useless/untrained?

Helpful to know though thanks, On your slow speed I would have expected the headloss to drop quite a lot especially as your filter is quite big.