Hi,
Yes, there are pages and pages of input current data in the base PIC data sheets, but I often find it difficult to determine which "optional" internal modules are active, particularly in the PICaxe environment. In addition to the Brownout detector (maybe a false economy to disable it), the FVR, Watchdog (aka Sleep) timer and PLL modules may be worthy of examination (the Comparators are presumably off by default in an M2).
Your measurements are useful and highlight that lowering the frequency doesn't save as much current as might be hoped. So for a really "aggressive" power saving method, it might be worth
raising the clock speed to 8 or 16 MHz to perform any significant calculations in a burst of activity, and then Sleeping for the remainder of the time.
In addition to minimising the current it may be worth minimising the supply voltage, although a PWM buck converter might consume more power than it saves. It seems slightly "strange" (and inconvenient) that the 08M2 has a significantly higher supply voltage requirement than the other M2s.
Many microcontrollers (from other manufacturers) rely heavily on (constant) current sources so the power supply current may be largely independent of the supply voltage (the input
power will of course still increase with the voltage). However, the Microchip resistors (e.g. the Weak Pullups) actually
look like resistors (i.e. they have a linear V/I characteristic) but I don't know if this has any significant bearing on the overall power consumption. FWIW, it may be safer to pull unused inputs High with the WPUs (or even the rather puny High pin-drivers) than pull them Low, which may destroy the Low drivers, if accidentally connected to the supply rail.
Finally on the topic of lowering the clock frequency (below 4 MHz): Note that for some instructions the "Watchdog" timer may cut in at 1 MHz and below, spontaneously resetting the PICaxe. Also note that the "base" PIC has
two "31 kHz" oscillators, one derived from the "normal" clock and the other from the Watchdog timer (which is around 100 times less accurate, and I believe is the one used by PICaxe ).
Cheers, Alan.