Part of the issue with the WPPSI is that it's intended for a narrow age range, and your DD is already close to the top of that range in ability.

There would be more headroom on a test with a broader age range (if she takes it while still fairly young.)

The achievement testing on kids this young is hard to get a handle on because a kid that's reading and doing math at all looks quite unusual. You probably won't get meaningful grade equivalents from the achievement testing.