So because I have no life I tried to test the actual impact of the hausa gather rate
Test 1:
24 hausa vils with grannary vs 24 british vils for about 10 mins
its a about equivalent, there were minor variations due to the fact that I couldnt set the african civ AI to just collect the res ( they start doing stupid stuff)
At first I saw that the euro vils were gathering more but i discovered this was because I didn’t pack them fully (24 vils on 4 mills). When it was 24 vils on 3 mills the efficiency was the same
Test2: same as 1 except hausa vils have +10% speed from market tech
there might be a tiny tiny effect like 0.5% difference
also looking at the idle villager graphs I think its clear why the net gather rates are more similar
on average about 4-5 euro vils are idling due to all the bumping and moving around
hausa vils are only idling like 2-3
So from that the net hausa food eco is probably comparable to a civ that is missing food silo.
Savegame of the scenario
hausa villager test with speed.age3Ysav (2.7 MB)
edit: if you want to test this scenario yourself, you have to set the hausa vils to gather from the fields manually around the 30-38 seconds mark