I dislike this idea to be honest. Because using tab in the address bar has always served another purpose: moving the focus away from the address bar onto other areas. This isn't only something within Opera but most other browsers (FF, MSIE) follow this same principle. So I can't help wonder if this wouldn't make things a lot more confusing.
Not at all. The only instance where TAB would not jump away if you are switching between website address and search mode (which involves you typing in the keyword in the first place). In that sense, it is shifting focus elsewhere - from the address bar to the search bar. Only that their position is the same.
Besides; alt-left and alt-right already allow for this behaviour (selecting other search icons).
I wasn't asking for TAB to switch between search engines, but thanks for that tip, this makes it faster than Chromium.