With 12ft/lbs rifles then its shot placement be it any of the usual calibre's. Nothing else matters.
.20 adds little to any equation not covered between .177 or .22, and the fact finding a pellet that a rifle likes is much harder in .20 unless lucky.
So like many before you, you are searching for a solution that isn't really any solution at all.
If you fancy having a .20 then have one, but its not for any of the arguments speculated.