First of all, you haven't specified whether you were doing OG Quant questions from the easy bin (first half) or the hard bin(last half). If you were doing the easy bin then the OG questions are way more easier than MGMAT CAT as well as actual GMAT questions.

I have taken all MGMAT CAT's. The questions are tough but not tougher than actual GMAT. The questions force you to think. 90% of the questions match the difficulty level of GMAT though 10% require too much work to solve.

But as I said, these questions make you think and make you adept at deciphering lengthy problems and finding smart ways of solving them. So don't worry too much. Just make sure that you finish all the 6 CAT's and repeats(if you want to) 3-4 weeks before the test. Last few weeks only take GMAT Prep exams. You can easily take GMAT Prep twice (that makes it 4 tests) without too much repetition. After that you can still repeat GMAT Prep and get new Quant questions, though Verbal will get repeated and wouldn't be of much use. So that makes it 6 tests in effect.

But i would recommend you don't take MGMAT's in the last 3-4 weeks as you would need to start get used to GMATPrep questions.

So don't get flustered, do the MGMAT's patiently and read every explanation. They are excellent practice. I took the six CAT's and they definitely helped me improve my thought process and score.

All the best!