A drive to a store to buy oil has a larger co2 footprint than the butter producing cow. That make the butter eating cyclist a more ethical person than a oil buying driver, if we are going to argue co2 footprints.
CH4 + 2O2 -photons---> C02 + 2H20
I'm not sure what time scale of heat per mass equivalencies are based on in your example; there is approximately 28 : 1 heat/mass eq. ratio at the 100 year scale, during which time the heating effect decreases exponentially with the decrease in methane concentration[0]. After 100 years the ratio is approximately 1:1 since almost all methane has decomposed.
A possible solution would be methane fixing bacteria (methanotrophs) in the guts of cows or in the ocean where > 75% of the current concentration atmospheric methane originates. The ocean also contains a significant amount in solid form that gets released as temps rise. [0]
[0]http://www.washington.edu/news/2014/12/09/warmer-pacific-oce...
https://en.wikipedia.org/wiki/Dairy_cattle lists a few sources for the 4 years figure.
Not to say that I am not a fan of less selective counting. I in big favor of counting footprint based on a person complete effect on the environment rather than specific choices. If we counted everything from purchases of items, clothes, travel, commuting, diet, and so on, I am convinced that the end result is more enlightening than just looking one diet item vs an other.