Research suggests that it is the more average children who get the most out of a grammar education.
Why not just put her in for the test and see how that goes? To be honest, if she's out of catchment, it's a long shot.
I reckon I'd be looking more at factors such as distance, amenities, opportunities. What's on offer at your local school? how do they organise who gets to do which GCSEs? Does your local school offer outside-of-curriculum opportunities? If yes, what? And how are they shared out?
I've started on a tour of secondaries. I was quite ... surprised ... by the amenities in the grammar schools. And by the extra-curricular stuff. It's all so bright and shiny, and there's so much of it. In our local comprehensive, there is less, and it's a bit tattered and battered. Fewer trips, fewer places; fewer drama productions; fewer orchestras, etc.
But, on the other hand, it's all very close. Would my dd really fancy staying late to take part in all this amazing stuff if there's a long, cold trip home at the end of it, through rain, dark and complicated transport links?
Academically, I suspect top sets in any comprehensive are pretty good. But I have no idea, really. I'm not a teacher, and I'm not one of the children in any of these schools.
Good luck with your decision-making.