At Veritas, we have definitely noticed that the 800 score verbal portion gives out higher scores than the GMATPrep or the Official exam. The questions are less - what should I say - "deceptive" than on the GMAT and you get a higher score from the 800 Score for a given performance. It is tough to say exactly what the corresponding score would be on the GMATPrep but would almost certainly be lower. Of course the questions on the 800 score are also not edited as well, so you may find the official GMAT questions more to your liking! The quant is more reasonable in terms of the score but, like all unofficial tests cannot be used to predict your actual score.

Only one way to know how you might score on the test, take the GMATPrep and see how you do!

Good luck!