Current consumption in MCU
I'm using the F051 in two projects and found the datasheet values to be fairly accurate, though I haven't done a full inspection like I did with other micros.

My sample points are very limited but were taken with just a minimal set of peripherals active (sum of Ivdd and Ivdda):


Using the internal oscillator:


1 MHz: 0.96 mA
0.5 MHz: 0.75 mA

HSE in bypass:


1 MHz: 0.79 mA
0.5 MHz: 0.54 mA



And the values in the datasheet are pretty close to that. It is a bit of a mess to find the values which are realistic but the table of typical values in run mode with data processing running from flash is quite usable:



So this one tells you that with peripherals disabled (that is close to your use-case) you'd get around 8 mA out from reducing the frequency from 48 MHz to 8 MHz.

The ADC is a bit of a wasteful component which I'm not sure how it scales with frequency, but I suspect it doesn't scale very good.