As much as I wish it was, GMATPrep is not meant to be a prediction of performance, but rather a measure of preparation. It's the same for other tests. Take some Kaplan tests (maybe just not right before your actual GMAT). Kaplan scores seem a bit lower than actual performance. If you are in the 600's, great. Take the free MGMAT test online (maybe without the timer due to long math problems). My score on MGMAT is similar to my score on GMATPrep (MGMAT is just 10-20 points lower). Princeton Review doesn't seem too bad either from my experience. I actually enjoy continually taking many prep tests, because it really makes me focus on timing.

Anyway, there is alot of information on this board and others about prep test performance versus actual test performance. This can provide a sort of gauge for readiness.

Lastly, I doubt that you recognizing problems had a huge impact. Unless you were able to identify the answer without really thinking about it, but just from memory.

I've seen it suggested on the forums that if you come across a familiar problem on the practice test to go ahead and let the timer run for the full two minutes, even if it doesn't take you that long to answer the question. By doing so, you will make sure that you aren't giving yourself a timing advantage on the practice test that you wouldn't get on the real test.