I'd assume the Stim and buff would not stack in your scenario because leveling pets is already pretty fast. I think the Profession most affected would be Chef. They create specific supplements for a specific group of pets, so the second you're able to get an entertainer buff to cover pet XP generically, there is no reason for a chef to make supplements anymore - so then when a CH can't find an entertainer, they're also not going to be able to find supplements. You kind of end up messing up a quite a few things pursuing this.