I think I just found something big for the wiki.
In this gen, it appears that the exp. yields can go above 255.
Here are a list of them, by index number:
As you can see, Arceus now has a base EXP. yield of 324. And Reshiram and Zekrom have a base yield of 306 each. (Also, Blissey has a yield of 608. o_O)
And I was wondering why the yields seemed so low for the starters. It's because in this gen, they have a base yield of 28.