I think athletics was nerfed in the last patch. I find that I'm among the fastest on the field in medium armour and 5-6 agility, often racing on par with the agi-based ninjas. After a certain point there doesn't seem to be much of an effect at all. Do other folks have any proper stats on this?
I finally found my favorite mix number wise, this build. its almost at 30, where it'll be 15/24 str/agi, 5 ps 8 ath, 8 wm. I've found getting the max out of my foot speed requires being very strict about equipment loadouts. I wear a rabati, tribal warrior outfit, sarranid leather boots and leather gloves . 3 pound spiked mace and light crossbow, one bag of bolts. I switched to boots down from splinted leather greaves over mail (1.5kg difference) and the light crossbow from regular crossbow (.5 kg difference), and that little amount was what was holding me back from running at what felt like 7 athletics.
So maybe encumbrance is a big issue with seeing full agi benefits.