IMO, what would the value be in studying any longer? all of your tests are well above your intended score, so what would you look to add by not taking it as soon as possible?
the gmat preptests are generally the most accurate; i'm not sure where you read that the actual GMAT is tougher. most people believe that the gmat preptest scores are about +/- 30 points from your actual score. if i were you i would take it as soon as possible.
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