"our corporate rates are already lower than most in the USA"

Given that the U.S. has the highest corporate tax rate in the developed world this isn't exactly saying much.

The combined corporate tax rate in Ontario, at 26.5%, is slightly higher than both the average (mean) and median tax rate among developed countries, though not by much.  Last I checked the mean was about 26.0% and the median was about 25.0%.  Probably not a big deterrent but also not an incentive.

Ohh, and corporations are DEFINITELY leaving the U.S. due to their high corporate tax rate.  That is why Burger King merged with Tim Hortons to move their headquarters to Canada and why Pfizer almost moved their headquarters to the U.K. before new rules blocked them.  Many more corporations simply never setup in the U.S. from the get go.