I'm not going to post numbers b/c there is no cut off point at which you should try to transfer...I will give this advice though:
There is NO magic number to get in to a US school - there were applicants that outscored me on Step 1 by more than 15-20 points who were not accepted or were wait-listed. The previous post of needing a 245 is not true. This is GOOD news - it means these schools are looking at more than test scores.
One thing that will help you is to make sure you are performing well in any Ross clinical rotation that you may be in while waiting to hear from schools. I had the directer of medicine at my site write one of my letters - he had worked with students from Hopkins, Mayo, and Yale and was able to compare my performance to theirs - this definitely helped. I also had my undergraduate professors update my letters (to include my Ross performance).
I really just want to emphasize that there is no special combo of grades/scores/activities that will get you a transfer spot...some of it is dumb luck (do they have spots the year you want to transfer) but most of it is showing your interest in medicine and proving that you have the ability to pass their 3rd/4th year. Last point - I am totally excited to be in the US system now - but I know things would have worked out for me at Ross too....not transferring is not the end of the world