It's certainly a constraint on raising taxes when you reduce the base; you get people who leave the country, or go from one province to the other. There's not a whole lot of evidence about the responsiveness of raising the rate in terms of international migration. There are many other things that influence international migration besides the top marginal tax rate.
After all, the top marginal tax rate only applies to a small part of the income that the person is earning. But I accept your point. One has to weigh the possible erosion of the base as a result of people fleeing the country when you raise the top rate versus the fact that a lot of income that people get at the top is a result of luck rather than hard work. I think capital gains partly fits into that category, for example, capital gains on housing.