My real DAT was a little easier than the Topscore and way way easier than DATachiever. I was getting 19-20 on Topscore and 17-19 on DATachiever. And ended up with a 21 on the real test.
A lot of it depends on your particulat test and your math skills, I hate complicated stats problems and don't have too much problem on trig, so not having probability questions beyond simple heands or tails helped me.
Overall the practive tests are a good inicator, so do as many as you can.