The 3.73 may reduce the MPG a bit, but honestly not enough to notice. You're already drivng a 5.0 V8, so you're not going to be impressed with the mileage.
The Brembo package includes summer tires, so that may be something to consider. Being in Canada I'm sure you're aware of winter/summer tire pros and cons. I'd keep the summer tires on for max grip and change to winter tires, or store the car for the winter. Not sure of your situation. Both of those options require more money to be spent on extra tires/wheels or a winter beater. Or you could stick with a set of "no-seasons".
The 19 wheels will increase the stiffness of the ride and are more prone to bending when hitting pot holes. Of course, they look great!
The brake package can be fun for you even on the street. Do you plan to do some back road running once in awhile?
Just some things to consider.
13 Ford Mustang GT Track Pack 6MT (my DD)
06 BMW Z4M Roadster (weekends)
97 BMW M3 (Race Car)
99 BMW Z3M Coupe (back-up race car)
85 BMW M635CSI (Show)
10 Subaru Outback 3.6 (wifeys DD)
08 Chevy Duramax 4WD (tow)