No, receiving the top scaled score does not equate to a perfect raw score (full credit on all items). It means that the student is at the top of the ability of the test to spread the population in a meaningful way--IOW, has reached the measurement limits (ceiling) of the test. The suggestion of using the next year up test is reasonable, as it is not possible to distinguish a child who is (let's say) +4 SD from one who is +5 SD, if the scaled scores top out at +4 SD. Although no true scaled scores could be obtained on out-of-level testing, it would certainly help in advocacy for grade-skipping. Ideally, one would want to keep moving up the grade levels in out-of-level testing in each subject, until his scaled score approached +1 SD of the mean, or so, at which point it might be possible to identify growth/need areas, as well as personal strengths.
No one can predict whether his performance will be just as strong in the future, though it seems reasonable to expect him to test well, absent any specific injurious occurrence.
A skip decision, especially for a young five year old, naturally must include many considerations, in addition to academic/cognitive readiness, such as those you have listed.
