Like Kaplan, expect on average to drop a few points below what you're scoring in Topscore. The real thing is most similar to Topscore, but the difficulty lies between Achiever & Topscore. If you're pulling atleast 15+, but preferably 17+ on Achiever, you'll rape the real thing 🙂
I never took the Kaplan Course but the Top Score and DAT Achiever exams were much harder than the real DAT exam. Top Score being closer to the DAT than DAT Achiever.
I'd say the PAT section on the real test is slightly harder than topscore and much harder than Kaplan. However, the subject sections are similiar if not easier on the real test than on topscore and about the same as kaplan.