To give you some optimism--I took the Kaplan diagnostic test last October & scored a 460 (with no preparation). Last weekend I took the GMAT and scored a 690 (in fact I'm scheduled to take the test again on June 19th b/c I'm confident that I can break through the 700 level). And in response to dan.gmat.gre--I did all of this while working 50-60 hours a week at a very involved job. Therefore, the kind of improvement you're looking for is most definitely possible. But I can assure you that moving from the 500 level to the 700 level is no quick & easy task.
I would do as the above posters mentioned. Take a step back from the CATs and go through each MGMT book with a high degree of focus and attention to detail.
You should start by going through the number properties & word translation books in great depth before starting back on the CATs. If you go through every word of those two books and do every single practice problem--to a point where you understand every single concept, I can assure you that your subsequent CAT scores will greatly improve. Those two books in particular (as well as MGMT's SC book) are pretty amazing imo. They do a remarkable job at explaining the most challenging concepts on the GMAT in a clear and concise manner. If you can do every problem (and every problem referenced) in those books you'll literally be over prepared for the exam.
Doing CATs over and over won't get you the score you want--trust me, this is what I did for several months before realizing that I needed to step back and work on the fundamentals. You need to understand every concept to the highest degree and then develop strategies for how to handle each type of problem. The practice CATs should really just be a way for you to practice your timing and learn to deal with a wide variety of topics in one long 4 hour sitting.
It's also worth noting the importance of keeping refreshed with material that you have already conquered. Even after you're confident that you have mastered a particular area, don't just forget about that topic and consider it done--review it, at least briefly once a week so that you won't forget it. There's nothing worse than feeling confident in your mastery of a topic only to be stumped by that topic a few weeks later b/c you haven't been reviewing it. One of my methods: I have ~200 flashcards I review on a daily basis just in order to ensure I don't relapse on any material.
And on a side note, you mention that you are using MGMT & Princeton books but using the Kaplan CATs. Are you enrolled in the Kaplan program, or did you just purchase the practice tests? Reason I ask, I've gone through all of the Kaplan CATs and MGMT CATs and can assure you that the MGMT tests will prepare you to a much higher degree than Kaplan's. Furthermore, the scoring algorithm in Kaplan is garbage imo and will give you an unrealistic score every time. I took the Kaplan in classroom course first and then went through MGMT's books on my own. I cannot reiterate enough how much better the MGMT program is, I wish I would have saved my money and started with MGMT.
Good luck,
Michael