Grand Prix is the undisputed busiest time for agencies, even more so than during the Jazz Festival, Les Francopholies, Just for Laughs, and other big events in Montreal.
I know of several reputable agencies that would have quadruple the amount of ladies working during this time than during the "normal" season.
For example, an agency that typically has like 6-8 girls working per night could easily have 17 to 20 + girls working per night. In fact, some girls have been known to just make appearances at agencies and were never heard of or from again.
So, there are two sides to the coin: 1.) you might not be able to see some of your favorites or even at the times you so desired because there will be so many other hobbyists in town
On other hand, 2.) you may be lucky and get to see some special or undiscovered gems that you would not normally be able to see as they specifically work just during this time to make some quick cash.