That's comparing apples to oranges. Totally different in terms of cores, hyperthreads, ACTUAL TDP and also there is a lot more to it than just that. What motherboard and chipset are used can be factors, what the VRM configuration on the motherboard is, etc.
Strictly speaking off of pure TDP performance though, I'd say the U12S is too small. It might "work" and would certainly be better than the stock cooler, but honestly, that's not saying much. The U12S IS a good cooler, but not the one you want to use for this CPU I don't think.
Now, the U14S on the other hand, performs within about two degrees of the D15 in most reviews, is MUCH smaller, being only about half the size since it only has a single finstack, and is pretty capable...