I just stopped fretting over it and ordered an Intellifaucet D250 and filter kit... with the low-flow option. What this does is alter the control program so it's more accurate at low flow rates. I guess it reduces the expected temperature change in response to valve adjustments to eliminate self-induced oscillation -- which for higher flow rates means it might take a fraction of a second longer to home in on the temperature.

Tomorrow I've got to call the plumber for a consultation. I'll definitely want feed valves before the filters so I can service them. I should also check with him if I need vacuum breakers, but I suspect probably not. I can probably break a lock by unscrewing either or both filter canisters. But if they're cheaper than me I might have a set installed anyway.