GMATPrep is the most accurate indicator of your actual GMAT performance on the market. Unlike practice tests prepared by GMAT prep companies, GMATPrep features real but retired GMAT questions.

If you got a 780 cold, then you may be in the 0.001% of GMAT test takers who are simply naturals. I'd still spend some time prepping, perhaps minimally in your case, and take the second GMATPrep. If you get a similar score again, then take the real GMAT ASAP.

By the way, I'm super jealous.

Hahaha, like the fact that you're growing a lucky mustache.

Sounds like you are making some good progress. Check out the GMAT Flashcards if you haven't already: http://www.beatthegmat.com/mba/free-gmat-prep-flashcards

There is some coverage of much vs many that could help.

Good luck!

Just a quick question for those out there who have taken the GMATPrep tests, Manhattan CATs, and the actual test.

So far I've taken two GMATPrep tests, and I haven't got a CR question wrong, along with only missing 1 RC.

However, on the Manhattan tests, I'm only at around 60% on the CR and 70% on the RC.

So, is this discrepancy merely because Manhattan cannot replicate true GMAT style questions, or is the difficulty of the GMATPrep questions not indicative of the difficulty of the actual test?

This was answered in the 2nd post of the thread. The GMAT Preptest is the most accurate representation of your score. All others have a larger variation, and could contain either harder or easier questions. It's commonly thought that the Quant on MGMAT, for example, is more difficult than the real thing.