They'll be using chromium to some extend, that doesn't mean it's going to be a chromium browser that's exactly like Chrome, Yandex, Torch, SRWare and what have you. That is what you seem to imply.

As you probably know "some stuff on top of that" is rather broad in Opera's case, assuming they keep current functionality. If they have business sense they'll also want to keep their 300 million users, I doubt they'll do that by killing their product and making an Opera branded Chromium instead.

http://my.opera.com/ODIN/blog/300-million-users-and-move-to-...

It states that for the user, it's going to be the same browser. It also states Opera extensions will keep working, so at the very least we know they'll be making something not as chrome as the other chromium browsers.