Don't worry too much about
MGMAT timing. I have given 4
MGMAT, 1 PR and 1 GMAT Prep
Please don't look at my absolute timing below, just see the relative time for perspective.PR: I finished 30 mins before time with 0 incorrect. I talked to my X-roomie (GMAT-780) and he said actually main GMAT is not much harder than PR.
MGMAT: I have finished quant 5-20 mins before time in each. "I feel" like they add too much unnecessary computational complexity on top of the concepts (maybe for a good reason, but I personally think otherwise). As long as you have no conceptual problems, you are all set to rock GMAT.
GMATPrep: As we all know, this is the closest one can get to GMAT. My experience was that the questions were conceptually same as
MGMAT but with NO (or less) added computational complexity. Although my section timed out, because I spent 30sec-1min extra to reverse calculate each answer, as I wanted to try something different :p. But if I did normally, I should have had 10-15 mins spare in the end.
In sum, as long as you don't have conceptual issues in
MGMAT, I think you should be okay in main GMAT - again that's what I think

ps. I have not given GMAT, so I can't tell if real GMAT will be as hard as
MGMAT. But based on what I have heard, that's my take.