If you're question is - why is she only aiming for Grade 5s instead of Grade 7s?
Instead of - based on the following metrics what should she be capable of?
It seems like the answer basically is you've put her under too much pressure and she's fed up with it, so she's told you she only wants to try for Grade 5s to stop you going on about how much work is required to get the grades you want her to get to go to 6th form.
It's pretty obvious she's likely to get greater than Grade 5s if she's at a Grammar with higher expectations and you're making her do loads of homework. So the problem obviously isn't one you can fix by giving the right performance metrics.
You're not asking the right question - the question is - why would she say that to you? She's either said it because someone has made it seem too hard to get Grade 7s so she thinks she can't do it, in which case it's a confidence issue. Or because you've put too much pressure on her and she's said it to shut you down and make you back off.
She clearly knows what your expectations of her are, she's at a grammar with high expectations, so I doubt the problem here is low expectations. It's much more likely to be related to being sick of everyone having high expectations and wanting them to back off, or feeling low self confidence if everyone has reiterated how much work they'll need to do to get the expected grades.
It's not actually that hard to get Grade 7s even in a bog standard state school.
It's Year 9, there's a huge transition between year 9 and year 11. You'd do well to back off a bit now and stop talking about expected performance and just tell her she can only do the best she can do - because that's true. Be supportive of who she feels she is - I want you to sit down and do your homework every evening and try your best, that might mean you get Grade 5s, but you might surprise yourself and do better.