Hi Shane,
The
MGMAT tests are definitely challenging, and from what I have seen, you can expect a 30 point increase above what you score on them, when fully prepared for the GMAT.
However, they really help you develop time strategies, and I suggest you try to complete the rest of them in test environments. The quantitative section is especially challenging time-wise, since some of the problems can be rather convoluted. The verbal reasoning and comprehension questions can also be ambiguous, but still help you think like a test-taker.
After 2
MGMAT tests where I scored 720 and 730, while my average in another practice tests was closer to 760, I really sat down and reviewed the two tests to see where I was lacking. I managed to develop a quicker way to read through lengthy problems, and most importantly, to skip questions I have no idea about. This helped me get up to 770 in my third
MGMAT exam.
So my last word is that
MGMAT tests, though challenging, are really good practice.
As for verbal speed, my own strategy is to do my sentence correction questions really quick. I get the easie, shorter ones done in 20-30 seconds, and don't waste time double-taking. With CR's and RC's it is really important that you get into the habit of reading thoroughly once so that you don't have to keep going back and reading. Knewton calls that reading with "glazed eyes", and the only way to avoid that, sadly is practice. Another way is to take notes in your scatch paper while reading.
Good luck!