The
MGMAT CAT is really hard. I scored a 770 on the GMAT Prep but barely scraped a 700 on the first
MGMAT. A bit unorthodox, but I felt that
MGMAT SC didn't help me all that much. It just threw so much information at my face that I was finding it hard to decipher the meaningful stuff from it. It's still a great comprehensive resource, but perhaps a bit too detailed for my liking. I used Aristotle Prep's SC Grail and found that it actually made much more sense and was much easier to read through. It's only a PDF. If not, you can even refer the the Manhattan Review website where they have a free SC Review PDF that goes through all the main points. So I'd suggest skimming through that first and then going through
MGMAT for places where you need further clarification.
The OG might be good practice for you, but for additional practice you could consider the Veritas Prep Guides. They have a bunch of practice questions that are actually quite good. If not, try searching the forum by tags once you're done studying the strategy. Practice maketh perfect.
I had another form of focus issue. I used to finish my Quant section with 30 minutes to spare and then just get really bored on the Verbal and just start guessing answers to RC and CR without really paying attention to it (and I'd finish with like 20 minutes to spare and end up with a 670-700). I realized I was being stupid and just sucked it up and sat through the exam, as much as possible when I took the GMAT Prep. I took notes for RC and read each CR carefully and thoroughly. And I got a 47 on the Verbal, which is my highest till date.
I think that it's possible to get a 650 in your case, with the materials you mentioned provided you study in a focused and effective manner. Quality > Quantity