Welcome Beatnik! I too had a kid that tested at 99% in the Scantron Performance Series testing several years back, but it was in Math instead of Reading. SPS is an adaptive test that tests out of level, so you probably should pay attention to the actual Scaled Score in comparison with the testing group. What we had found is that my kid scored 150 points higher than the highest score of kids two years older than her. At that point we decided to do both achievement and ability testing, and ended up doing an additional grade skip in Math.
Your DD's other scores are not too shabby either, and the SPS scores do vary over time. That particular test is multiple choice, and it gets progressively harder with each correct answer and easier with each incorrect answer.
With regard to additional testing, we did find that very beneficial, as we were able to isolate relative weaknesses and gain insight on my DD's true abilities, as well as finding proper placement in school.
Gook luck!
