GMATPrep is the best indicator of what to expect on the real GMAT. Normally if you take a GMATPrep test, then a GMAT the next day, you'd expect a difference of 40 points or less between the two scores*. I agree that the difficulty level of the hardest current questions is higher than what you'll find in the OG, so it's a good idea to see as many difficult problems as you can in GMATPrep (by retaking the test).
*(the stats- if the standard error in GMAT scores is 30 points, as I've seen reported, the standard error of the difference is 42 points, so 2/3rds of the time the difference in two scores should be 40 points or less)
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