Hi Occamsrazor!
Kaplan tests have always been pretty accurate. Of course, many people report scores wildly different from their test-day results, but that's true for all practice tests--so many factors, from stress to noise to time of day, can impact your score that consistent results during practice are impossible.
However, our tests did seem to be scoring a few tens of points low, a situation that we swiftly corrected in our coding. Unfortunately, as of now that new scoring algorithm applies only to our classroom courses; we are working on incorporating this new technology into our published works as well, but haven't completed the transition yet.
In the end, though, no test-prep company, even Kaplan, will be as accurate a measure of your performance as the powerprep exams. I would consider the Kaplan CAT score a fair analysis, but any doubt can be cleared up with an official GMAC practice test.
Best of luck, and let me know if you have any other questions!