Anecdotally, it actually seems like the January results are the ones that take longer to come back. You'll still get them before Feb (hopefully!) but it might be 7 weeks instead of 6, for instance.

Beyond that, I don't see any real advantage of one date over the other unless you think that your child will learn a bunch btwn Jan and Feb and be better prepared in Feb. Interestingly, the 8th grade cohort with whom they are being compared takes the test in the first week of October (at least in CO).

Study Strategies for Accelerated Learners