If you want an outstanding score I personally advise waiting a few years before studying for the USMLE. I think you'll have a tough time because you'll be learning most of the stuff for the first time, as opposed to reviewing/revising the material.
I'm a final year medical student studying for the Step 1 and find most of the material relatively easy and quick to cover as it's all material i've learned before. I actually tried to study for step 1 when I was in first year, and gave up because I found learning lots of concepts from scratch completely overwhelming. It's amazing what difference a few years make... I believe the clinical years in med school help massively...
Sorry, it's probably not what you wanted to hear... But I'm sure if you're really determined you could do it!