If this is true, then something has changed since 40d. I spent a long time frustrated in a few forts in 40d because of how much slower magma went than water.
This test used 40d, and took place long before .31.x was released.
Two Experiments with a Major River, also under 40d, helps confirm the behavior of rivers themselves. It seems more likely to me that you spent a long time frustrated at how slow fluids diffuse in general.
Back on topic. The idea of computing with fluid levels pops up now and then, and I was trying to figure it out myself for a little while, but it's generally a terrible idea. Values of 1 evaporate (sorry nekoexmachina, even with magma), and pressure plates are still pretty binary; toggle on/off based on whether it is higher or lower than whatever fluid level.
The only effective way to split water into smaller values than 7/7 would be to isolate small areas, and then allow them to diffuse into larger areas. Each "cell" would have to have multiple pressure plates hooked up to determine the water level, and then the output would have to interpret what to
do with that information, which will probably just end up being essentially binary again; there isn't really a way to input "some" and equally output "some".
The sensitivity of controlling the water levels means you'd have to be incredibly precise, and even if you did get that working the isolation/diffusion process would inevitably render the whole thing incredibly slow.