@Lycan
They do eat a bit more, but usually the difference is not significant. I've actually written down some observations before evolving my pets, you can take a look:
Owluck lvl45 - 235 food, Royowl lvl45 - 279 food
Sawrotag lvl47 - 250 food, Pentadile lvl47 - 295 food
Unicubb lvl71 - 365 food, Ursufuzz lvl71 - 430 food

As you can see from these examples, evolved pets can eat anywhere between 45 more food and an extra half a pumpkin compared to their unevolved counterparts. I'm not sure, but I think the difference becomes more significant as your pet's level grows.

In my opinion it's worth it to have at least 2 evolved pets early on in the game. Not only do they bring twice as much PC, but their WU is higher, too. Be careful, though, as your pet changes color after evolution, so if you have a rare color you want to keep I'd suggest waiting for a better opportunity. I hope this helps.