If you're going to a DO school, then your best bet is getting advice from a DO student who has taken the USMLE and COMLEX.
For the USMLE exams my advice is pretty straightforward. Attack each subject as hard as you can during each class or rotation. If you *really* learn the material the first time you hit it then it will be a whole lot easier reviewing it for the step exams. For step I, I recommend taking shelf exams. A lot of MD schools use these exams for finals.
For 3rd year, I recommend always using the "smaller" version of the recommended text book. Read the Cecil's Essentials and not the big Cecil. I always went for an "essentials" type of book and found that I was able to get through the whole book a couple of times during each rotation. Defintiely read as hard as you can during rotations. It's tempting to just blow through rotations that you are not really that interested in but for the sake of the USMLE exams, you should bang out each rotation as hard as you can. (I did not do this but do as I say, and not as I do...)
Also, do as many practice type questions as possible e.g. Kaplan, NMS, etc... Some of my classmates were doing these questions from the beginning of 3rd year (tho I did not).
Basically, if you really attack the subjects the first time, then reviewing will be relatively painless.
As for correlations between MCAT and USMLE and SAT... I think that there is a reasonably good correlation in the sense that people who "do well on standardized tests" have the advantage. However, for the step 2 exam I think that your performance depends way more on your knowledge base and ability to apply that knowledge, than pure knowledge as in step I, and aptitude/potential as measured by the SAT.
I scored well above average on the SATs, above average on MCAT, Step I, and Step II. But, I know of a few people that were all over the place.
