You have already had quite a lot of practice. The OG diagnostic is rather helpful when you are at the beginning of the learning process and you want to see how much work you need to do in order to reach the desired level.

I would suggest going through the diagnostic test and timing yourself. However, if you do not manage to select any answer within the time limit (not guessing of course), I would make a note about it and then continue with the question. Later I would write down how much time the question actually took.

This way you will be able to measure your progress and you will check whether you problems to deal with any question type within the assigned time limit.

Thanks, Kasia. So it seems like you think the value I'd get from the OG diagnostic will mainly just be in working on my timing, which I could probably do just as easily with another practice test, right?

Do you know what difficulty level the problems in the diagnostic are? Quant is my weak area. I took the first GMAT Prep test last week and scored a 40 on quant. My goal is to raise that to around 48 or 49. Since the questions in the diagnostic obviously are adaptive, will there be a mix of question difficulty levels, or should I expect most of the questions to be below, at, or above my current level?