So I am not sure if anyone has experienced this yet. However just as the title says, it is not stacking with the Cake.
I was using crafted xp pots only. I did not test the xp scrolls and i also do not have the pledge of mara.
This was tested on level 42 mobs. They give 554 xp without any xp boosters
@ 50% you're getting an extra 277xp from xp pot only -> 831 total xp
@100% you're getting an extra 554xp from cake only -> 1108 total xp
@150% you're getting an extra 831xp from pot+cake -> 1385 total xp
However you are only actually getting the 100% from the cake and you are not getting any xp from the pot. So save your pots for now.