You have nothing to loose by asking for the testing now. (Unless she has a sort of squirly personality, and is likely to 'mess with the tester's head just for the fun of it - in which case wait until she knows how to be serious at times.)
She'd be judged against other young 6 year olds when the score is calculated.
If she really is an outlier, and needs a gradeskip or several subject accelerations, I think that is easier when the peers and younger and more 'go with the flow.' I also think that kids who need to be skipped are usually (not always) happier socially with 'readiness-mates' as well.
Best Wishes,
Grinity
