I actually ran some tests tonight.
I used same placement of deer, ox cart, and 5 starting gatherers for each 2 minute test.
Thor without pig sticker .85 / gatherer / second gained 375 food (tested twice exactly the same) This was my base rate of 0% bonus. They don't make 510 food because of bumping, dropping animal, and carrying capacity limitation.
Odin with 5 gatherers for a couple of minutes tried this a couple of times was 11% and 14% more than the 375 base test. (so more than 10% but less than 20%)
Thor with pig sticker a couple of times worked out to 11% and 17% (so more than 10% but less than 20% again)
Thor with hunting dogs but without pig sticker I did once and it was 24% more. (less than the 30% bonus that file indicates)
The tests are not entirely conclusive either way, but I am sure if you try labourers gathering from farm with (+22%) plow upgrade and without, you will not see 22% bonus, it will be a slightly smaller margin, but still relatively accurate. I don't know why, but all gathering bonuses seem to be slightly less.