Probably pretty boring to do, but try beating the same wild Pokemon in a range of levels (Level 3, 4, 5, 6, 7, etc Minezumi) with one of your Pokemon. Beat levels 3-7 with your Pokemon being level 3, then do 3-7 again with your Pokemon being level 4, then at level 5, etc. List the experience you gain for each win at each level. Make sure you never use two Pokemon in one fight.Ok, I finally managed to beat that Level 6'
Only the Level 7 is left, I'm trying to get that done. Updated the post.
Currently the multiples of difference between them has been:
You'll probably have to soft reset a lot to not level your pokemon (hence the boring), then rare candy to beat the level range again, and so on.
This way we can see if the experience gain boost is constant across levels for beating a pokemon 1, 2, 3, etc. levels over/under your Pokemon.