My DS10 has been ahead of his 5th Grade Math classmates in speed of learning in typical Math taught. When students, like this switch to Singapore Math, do they typically still remain ahead of their peers?
I would tend to think that a fast math learner would continue to learn quickly on any math curriculum, as long as it wasn't defective and chock-full of repetition. Singapore Math has a separate workbook for each half-year that you can skip, in part or in full, to keep interest higher and allow for a faster pace, and you can in addition skip some of the drill in the textbook itself.
Is there a way to pretest with Singapore Math? Or adjust the pace for quicker learners?
There are no pretests built into Singapore Math, as far as I know (ETA: I'd forgotten about the level assessments that are linked to by LittleCherub below). What I do with DS6 is assess how well he knows material we encounter, and throttle the number of skipped questions up or down accordingly (or alternately let him do them in his head instead of on paper to save time).
The amount of time taken for a section or page depends on the number and type of problems. At the end of sections and subsections are reviews, where there are a lot of problems on a page. I would estimate that DS6 averages 3-5 minutes per page in the textbook, not counting enrichment from the teacher's guide.
He tested 50% in 6th grade Singapore Math that he will do for next year. School thinks they should not move him up another grade.
I guess that I'd agree that 50% knowledge is not mastery, but the material in the Singapore books is condensed enough so that he could probably clear at least sixth grade over the summer with no sweat. Then maybe you could re-make your case based on beginning-of-year increased MAP scores or similar.