I dunno. The ATmega328p is a "Picopower" offering, designed to have very low power consumption. I haven't paid much attention to how competitive it has stayed with newer chips, but my general impression is that chips pretty much reached the stage where their power consumption (with suitable modes used/etc) was below the leakage current of most battery technology anyway (well, for GOOD batteries. Which would be another problem.)
The "modes" turn out to be really important, hard to measure, and even harder to predict between multiple CPU families. Newer AVR chips have "Core independent peripherals", which permit more to be done without waking the CPU from sleep modes - apparently that can improve overall power usage in many cases. But it's all a pretty black art - maybe just lowering the clock speed IS easiest, if it gets you the sort of battery lifetime that you're looking for.
(But I'd certainly be looking at the newer Mega-0 and "X-tiny" chips in the avr range, and maybe MSP430 (supported by "Energia") and ARM chips, as well as "classic" Uno chips (which are ~10y old now...))
Flounder, you've been coming here occasionally since 2014, complaining about how Arduino is missing some feature that you consider important, but no one else seems to care very much about. And neither the community nor the Arduino Company has been very sympathetic or helpful. You're clearly not the "target audience." Why haven't you switched to something else? (or, you know, written your own low-power core? I mean, really, you don't even want to have a 1ms clock-tick, do you?)
Or for or the sort of basic no-pwm, no-uart case where the only real timing-dependent bits are delay(), millis() and micros(), can't you just write your own wrapper functions that keep track of whether you've switched clock rates?
I have a device that will measure actual power consumption over time (available from Digi-Key, Part #1490-NRF-PPK2-ND)
Interesting. Microchip has a "Power Debugger" board as well (at over 2x the price of the Nordic board you reference.) I'd be interested in hearing how well the Nordic tool works with non-nordic chips.