Speaking from my own experience, at Université de Montréal (one of the top schools in music in Canada, along with UofT and McGill
) for Bachelor's degree in composition, you DO have to have some ability with an instrument, as one of the course requirements is ensemble-playing (chamber music). I'm not really sure to what extent your performance ability enters into line with your acceptance. I know a bunch of people ended up being in the "massed chorus" rather than in a chamber ensemble. And as far as I can recall, orchestra was reserved for performance majors.
Once you get to Master's degree, however, there are no course requirements for performance.
It also partially depends on the requirements of the teacher with whom you want to study. I know that Professor Belkin (my mentor) expected a certain degree of pianistic abiliity from any of his comp. students.