Forgive the long writeup.
All is well. I booted into the BIOS. The pump was running at a lazy 850 RPM (PWM Smart fan curve) but the flow meter still wasn't turning. No problem, it's due I'm sure to the intertia c-attack spoke of. I launched Windows, and opened HWMonitor to watch GPU load and temp, and also the pump speed. I ran a 4K full screen Kombuster stress test for 30 minutes and temps were about 5°C lower than before the radiator upgrade. So the radiator with the new fans made a difference.
Max pump speed was still a bit low at 1500 RPM. I ran MSFS2020 in 4K with DLSS DLAA enabled, which really stresses this 3090 Ti (constant 99% load, 440W) and while core GPU temp had gone down from 78°C to 73°C I wanted a little more cooling. I could have played around with the pump curve in BIOS, but decided to move it from the motherboard header to iCue, trying all the different profiles. With the 'Hydro-X XD5 Pump' profile, and Extreme profile on both rad fans I found that with MSFS running I got core temp down to 68°C and a hotspot temp max 83°C. Much better than the 78°/90° before the swap. Pump runs at a pretty constant 2300 RPM with that profile, and the fans a pretty constant 1300 RPM each. The system is a bit noisier, but I can live with that.
Kind of a side note: When I swapped the radiator I also moved the spinny thing from the radiator outlet to the waterblock inlet side. I would have thought that alone would make the flowmeter spin faster, since it was directly inline with the pump outlet. It was the pump speed curve in BIOS that was fooling me. I did however notice yesterday that the vane on the flowmeter seems backwards. I must have flipped the in and out ports around when I moved it. I wondered if maybe that's contributing to the way the wheel turning less vigorously, so I looked up the manual for the EKWB flowmeter. It said that any port can be used as an inlet or an outlet. I thought about draining the loop and flipping it over, but no way I'm wasting $20 worth of coolant if the temps are good now. Maybe I'll flip it around when I flush/fill in a year.
Thanks for the help.