I'm in a competitive specialty and was talking to the residency program director about this and that's basically the concern. With step 1 p/f then there is going to be more weight on step 2 (because at the end of day when you have hundreds of stellar candidates for a couple spots, you need something to help). The issue with that, of course, is that at least if you bomb step 1 you have two years to adapt. If you want a competitive speciality, take a research year and stuff, then bomb step 2 you have very little time to completely rearrange things as a 4th year.

Study for step 1 like it isn't pass fail so you have good foundation. Then work hard through 3rd year, study for each shelf. Then take step 2 as usual. Your "pre-study" is all the work for the exams before hand.