Interesting thread and very nice block. One thing about the design has me curious. I'm sure the top o-ring prevents leaks, but does it prevent "cross-talk"? Have you considered isolating the 3 holes? I was wondering about o-rings like such:
I realize it would be a lot of work changing it, but I'm curious if the pressure added by the nozzle/slit is causing some of the coolant to channel between the 2 top plates and leaving the system without reaching the botom plate. I'm not criticizing your design, I just thought since you are obviously tweaking it for optimization, this might be worth looking at.
peace.
unloaded
PS: have you ever setup one of these blocks with 3/8" inlet and 1/4" outlets? I'd be curious how it would do like that. Your results with low flow seem to indicate it would still do pretty good with this configuration and a lot of us are still using 3/8" setups.