I took an MGMAT CAT test the day before my actual exam, and the score I got was very close to the GMAT Preptest exam I had taken a day earlier. Both were actually lower than my actual GMAT score. I think at this stage of the game, you are better off retaking a GMAT Preptest. The MGMAT CAT test would have been good to take a week or so ago so you can pinpoint what areas you are particularly weak in.
Another reason not to take the MGMAT now is that the quant section is pretty hard - harder than the GMAT Preptest in my opinion. Even knowing this, if you were to take the MGMAT and receive a low score, it could unnecessarily hurt your confidence going into the test. Instead, I'd say just retake one of the GMAT Preptests and make sure you have your timing correct i.e. don't finish a section with very little time left, and don't finish it with more than 5 mins left.
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