Haas..the bottom line is once you have gone through the OG - GMATPrep becomes slanted towards your knowledge of the questions you have already seen.
I would still say it is the most accurate indicator out there and if you are scoring high on the retests you are most likely seeing newer questions, at least in the quant section. If you know you have seen a specific number of questions before then I would definitely discount your score. If you only saw one or two that you remembered I wouldnt be too worried.
There are 4 exams in the GMATPRep software. You dont need to uninstall to get them all. There are 2 test #1's and 2 test #2's. They each have a particular quant question they begin with - so to unlock them you just restart the test until the first question is different then the one you have already taken. This is easier then uninstalling the entire software and reinstalling.
I also recently wrote to Pearson and I found out there are currently no plans to update the software, the question files for the exams, or the official guide. I am not sure how true this is but it seems like they think the practice material thats out is good enough to get the job done, and for many it appears to be.
If I may ask, what scores have you been getting on your retests compared to your original tries? And what did you get on your first GMAT compared to your GMATPrep scores?