Without knowing OP's data, it seems to me that it wouldn't take too long to achieve a high confidence level of whether the follower adds Gold% or not. Random monster spawns and runs that aren't identical do add a lot more variance to the equation, but if the following holds true: "I farmed long enough for my gph to stabilize such that my gph only changes by about 1-2k gold every 30mins." over a decent period of time and through similar runs then it's most likely true.