For most people, and myself included, scores go up 1 to 2 points on average per section on the real DAT. Of course that is not necesarily true for everyone and there is no guarantee that your score will be better as it all depends on the questions you get, but your scores will likely be better on the real DAT.
So yes, if you average an 18 on your Topscore DATs then you will probably get a 19 or 20 on your real test.
Also, I don't think that rule applies to the PAT section though. What you score on Topscore should be pretty close to what you will score on the DAT.