i think it's dangerous to make the assumption that if you take Kaplan
CATs your score will increase 50-100 pts.
While the Kaplan
CATs are generally more difficult than most, you still need to understand the more basic questions as well. I personally would rather study from real GMAT questions and take CATs with real GMAT questions and score high than score low with Kaplan
and say "it's ok, cause those tests are hard."
What I've learned the hard way is that you need to build a solid understanding of the basics and work your way up to the more difficult problems. Manhattan GMAT books
do a great job of this by breaking each component down to it's smallest part.