the speed3 calipers are no better, its just a different design. its a different carrier, caliper, rotor and pads. wouldn't be hard to change it all over but theres no point really. that cost of a brembo kit or stoptech kit would cost about the same. as stated start with a good set of pads and rotors, aswell as ss lines and get racing brake fluid.
oh and you can get by with 17s you just need a low offset. (i'm a mazda tech and one of our customers has 17 inch snowtires/wheels on his speed)