Thanks for the name drop for me too (!) chopsuey 😀
In Canada, the board is taken right before your graduate (i.e. the same timing as USMLE step II). However some schools (like U of Alberta) do give a comprehensive basic science exam before allowing students to move on to clinical rotations. I am not sure if McGill does the same thing. The American students there might not get time off "specifically" to study for the board, so you might have to plan well ahead in your 2nd year to time yourself (I believe that McGill operates on 1 1/2 years of basic science, followed by 1/2 year clinical exposure and then 2 full years of rotations; similar to what they have at Baylor).
Ultimately, it is how well your graduates do on residency application that count, not the USMLE step 1 scores.
As for being overwhelmed by clinical rotations, it depends on your med school. At my US med school, we were overwhelmed. I started off with surgery and had 1 single day off in 8 weeks (and that was, ironically, on labor day). The cardiac surgeon I followed did not care that I saw 3 CABG's already; he still wanted me to scrub into my 4th CABG case in that week). On the other hand, some other med schools do "baby" their students a bit more. They make sure students get 1 day off a week on surgery (or internal medicine) to rest and recuperate, and don't place too many patients on your shoulder. My friend at U of Alberta was on medicine, and unless he is on call, he gets both Sat and Sun off every week. So both types of med schools exist on both sides of the border. If rest and not being overwhelmed by patient load are important to you, then you might want to avoid the school that work you to death. But you should double check that carefully beforehand (and that's hard to find out unless you get to meet 3rd years while you interview). And even if you get to meet 3rd years, there are ALWAYS some who either want to appear real tough in front of you (so you can look at them in awe) or are workaholic and really feel that being in the hospital 110 hours a week is the best thing that has ever happend to them since their parents gave birth to them 😉