I got 590 on Kaplan and 780 on the real thing. The problem, however, is that the Kaplan exam bears no resemblence to the real thing. It's not that the Kaplan exam is more difficult, the Kaplan questions employ different (often flawed) logic and test for things not found on the real exam, while missing out on key elements. I will note that my Kaplan result was skewed because it was clear to me right away that the questions were flawed so I finished but didn't give it my full attention.
Also, keep in mind that the 53Q assigned by the Kaplan CAT is an illusion. The current maximum for either section on the real thing is 51.
The result you should pay attention to is the GMATprep score. The vast majority of people reporting here have achieved scores on the real thing in line with their GMATprep results.