In my experience, and from the various breakdowns I read over SDN... I've seen MANY posters who get 18-20 on the science sections from topscore and end up getting above 20s on the sciences...
I don't believe there is much relevance between topscore and real DAT.... I mean sure if your scoring 13's and 14's on topscore I guess that would indicate your just not ready at all, but after you pass a certain mark (guessing 17 and above), there is no telling what you'll get on the real DAT