Well I don't have any numbers to show you. However I am almost certain while comparing my fists of fire w/ and w/o my +13 fire potency/+20 earth potency caligae I see 0 difference in my damage.

Same goes for earth potency too. I think this was not implemented correctly... We can use the potency on weapons if someone actually wanted too, but no one does lol. I'd imagine most people are rolling around moogle or ifrit like me. Thus there has probably been 0 complaints. If someone does have a pair of boosting feet please try it out yourselves and help me confirm that the potency is not boosting damage.