Ultramarina, I think your question is theoretically sound but perhaps only peripherally relevant in practice. Your DD's actual experience will depend more on the other students in her classes so you actually need to find out the selectivity of her projected math track. Disregarding that for the moment, I think that Pre-Algebra in 6th grade will be appropriate for a 95th-97th percentile math achiever.
For my non-mathy (but very smart) DD, Pre-Algebra in 6th grade has been excruciatingly slow due to too many students who shouldn't be in the class. At a result, the teacher goes at a snail's pace in order not to lose the bottom half. However, over the course of the year, some of those students have been re-routed to a lower level class and I believe that this pruning process continues over the next couple of years so that perhaps only about 15% will complete Geometry in 8th grade. In your DD's case, I think that she should be fine even if the track is limited to 99th percentile achievers, although in that case she may have to work harder than her classmates to keep up.
