I have to agree with womiwom about MGMAT's quant section. The questions definitely are a notch tougher than what you encounter on the GMATprep. That being said, I do recall encountering a couple of questions in my actual test that were on par, difficulty-wise, with the 700-800 level questions in MGMAT's CAT.
As for what you could do to improve quant, I will say definitely keep an error log
if you aren't already doing so, and re-visit the questions that you got wrong or had to guess in the CATs. Also, it helps to revise the math fundamentals from time to time. I see from your other thread you have the MGMAT set of 8: these books have quite a number of tips and tricks that are worth committing to memory (e.g. the 3:4:5 rule for a right angle triangle).
To improve your test-taking speed, I can only advise doing more practice questions to familiarize yourself with the various types of questions you will encounter during the test. If you want to, the forum's own set of questions are helpful, if very frustrating
. I hated those questions, but they forced me to know my math.
And of course, don't forget verbal, goes without saying.
All the best and good luck!