Hi @ThatSurfinOtter apologies for the late response.
I have checked this again with the development team and they have confirmed that you only get improvements to the feast buff when you first build the three farms. The additional upgrades do not improve the feast buff anymore.
I understand that it shows this on the screen each time you upgrade the farms but that text was only meant to relate to the building being built.
I apologise for any confusion this has caused and I have forwarded your feedback on to the development team.
If you have any further questions about this, please let me know.