I didn't do topscore, but I did a few Kaplan tests...
My midterm scores were off because I was only halfway through studying. I got an 18 Bio, 17 GC, and 18 OC
But these scores were closer to my test date:
Kaplan final (2 weeks before real one): Bio: 21, GC: 17, OC: 21
Kaplan Full length 1 (3 days before real one): Bio: 21, GC: 16, OC: 19
Real DAT: Bio 20, GC: 19, OC: 21
So I would say Bio and Orgo were pretty accurate, but I found Gen Chem to be easier on the real one.
Hope this helps!!