catkind "I'd imagine they'd say that they can't assess her as level 7a because they haven't covered all the level 7 material in class?"
You have a generous imagination 
For reference, last year David Didau wrote a blog post on the theme of problem solving based on a comp Y10 lesson they visited where the children all got shiny GCSE A*s in Y9 and they were working on senior, age 16-19, UKMT stuff (they were supposed to find it scary and struggle).
DD's comp is probably a bit above average and it's one of the random luckier years in terms of higher ability. They were given a 50 question L6 paper a few weeks into Y7, consolidated/patched the class L6-ness a bit, then moved on. L7 was well-covered by the end of the year.
"Question: if your DD was coming home saying how fun and challenging her maths lessons were, would you still worry about levels?"
No, subject to the lessons being credible maths-stuff which doesn't have to be NC. I can judge that reasonably well, but how does a typical parent determine whether the 'fun' is the right kind of fun? DD enjoyed maths in Y7, but that doesn't mean it was challenging or without some significant thumb-twiddling time. She's long accustomed to the latter and has one of those personalities that can absorb it.
Over the weekend DD restated her desire to do maths at university so we're kind of OK, but she'll eventually be competing with some lucky children who have been doing the above UKMT stuff since they were 14 etc.