Even in my experience, the Quant sections of MGMAT CATs were a bit tougher than the GMATPrep tests
and the actual GMAT. But, I think that you should not worry about that too much. You can always try GMATPrep test to know where you stand, w.r.t the real GMAT questions.
In my case, the MGMAT CATs had really helped me to adjust the pace. For the first two tests, I was finding myself short of time to attempt all the questions.
In addition to Quant, I found the RC section of MGMAT CATs difficult than the actual one. I think that they have designed it to be tougher.