I can't say for sure with the Pro-Kit but there is a good chance you'll need adjustable camber arms. With the H&R Sport springs my camber is around -2.5 degrees in the rear and it can't be adjusted since I don't have the camber arms. You could always skip it until after you have the springs installed and see where your alignment shop ends up setting your stuff.
One of the other Mazda3/MS3 mods, Rich, gave me some good advice... You can either spend $200 on the camber arms once, or $400-600 on a new set of tires every 5-10k miles (depending on how much camber you have and how soft your tires are).
So to answer your question, it is definitely going to be cheaper to just get the windows tinted. I agree with RS too as that is always one of my first "mods" to new cars.