Hi masci4,
Good work! Sounds like you're in a great position to rock the real test. With a 750 overall score you probably don't need to worry... but it could help to have a more balanced Quant/Verbal breakdown, so I'll say a couple of things to give you some direction.
First off, it sounds like you took the official GMATPrep test. If that's the case, then yes, your test is accurate and predictive. The official practice tests are composed of actual retired GMAT questions and use the same scoring metric as the real exam. My own students' actual test scores are usually quite close to those from their last GMATPrep test. If it was an unofficial test, be aware that your scores may be skewed one way or another. The testmaker's scoring algorithm is proprietary and their questions have a quality-control process (the use of experimental questions) that is unavailable to test-prep companies. This is not to say that unofficial practice tests aren't worthwhile -- they certainly are, especially to practice timing and strategies -- just that you should take the test scores with a grain of salt.
Schools don't look at computer adaptive test scores any differently than they used to do in the days of the old paper test. The differences are in the test administration and (of course) the test-taking experience. In effect you can miss more questions on the CAT vs. the paper test and end up with the same score. For example, in the scenario you outlined a student could get all their first-half questions right, go 50-50 the rest of the way, and still get a great score. This fact can be disorienting to students who are new to the CAT, but once you get used to it it can work in your favor. (I should also mention that a student could go 50-50 early in a section, but with a great run of all correct answers in the second half of the section also end up with a great score. This is less likely because of the test's adaptive nature, but it does demonstrate that you shouldn't spend extra time early in a section to "pump up" the test's estimate of your ability level. Although your score does take bigger jumps early in a section, this can backfire, as you will have less time for questions later in the section and your score can nosedive. Try to maintain a steady pace through each of the sections.)
As for your Quant prep, the OG 12 does have some hard math problems at the end of the Problem Solving and Data Sufficiency sections (they are arranged in rough order of difficulty). Because you were scoring well (even if not so well as on the Verbal), you were seeing mostly difficult questions on the practice test. Also be aware that the added time pressure of taking a practice test can make problems feel harder than they would if you were doing them individually. Although the OG has the best questions (by definition they are just like real questions), sometimes the explanations are too academic and ignore valuable shortcuts. The Math Review section in the OG is also dense and not especially helpful. So I would also recommend perusing some test-prep material. Our books, for example, have in-depth strategies on all the quant question types, our questions are modeled on official questions, and our explanations are geared to help the test-taker. Other providers also have some quality material, of course.
Hope that helps... great work once again!