The pop up for the boosted breed event said triple chance of getting "rares" and "super rares". It didn't say any extra chance of getting "ultra rares".
After getting loads of the common
rare Wizard when trying for Dark Angel, I switched breedings to ones which did not produce any rares, only commons and Ultra Rares and guess what, I only got commons and I spent quite some gold speeding up too.
My conclusion of the boosted breed after the event is it will produce extra rares and supers and this I think is why some people are really pleased with the results. But for those seeking the ultra rares, I don't think there is any increase in chances of getting them.