I went through the Kaplan 800 book near the end of my studies (I believe in the second to last week) and I was actually very impressed by the quant section (I only looked at quant in that book because I had read that the Kaplan Verbal material is not the best).
In my opinion, the strength of the Kaplan quant material in the 800 book is not the "difficulty" of the questions, but rather the presentation of the problem and the breakdown of the problem into smaller manageable parts that help to simplify the problem. In addition, the 800 presents a technique (for example, variables in choices) and provides several problems in different topics (e.g. percentages, word problems, etc) that illustrate how to effectively use that technique to master the problem. What the 800 book provides is not more "difficult" problems, but rather a toolset to help you destroy the questions in the 500-699 range. The tools can further be used on the 700+ questions, but at the 700+ range most questions take on a whole new level of complexity that the 800 book does not venture into.
As you and I have already discussed in our private messages Engin, the 500-600 range appears to be slowing you down on the cats so I recommend paying particular attention to how the 800 book presents the material instead of how "difficult" those questions are.
The 800 quant material is actually the book that spawned, and is the basis for, the ideas I discussed in my last blog "Quant Complexity."
