The sure thing front brake deal is to simply bolt on Box S rotors and calipers. It is a direct replacement and will improve your front brake heat capacity. Stopping distance is more a matter of weight and tire grip. No shortage of clamping power with stock Boxster brakes.
The rears are tougher. If you want Cayenne rotors on the rear and a wide track, what about using the whole Cayenne, rotor/caliper/hub/axle/ assy and cutting to fit your desired track width? You may need that heavier axle assy anyways due to the other "stuff" you've got going on with your car. I am afraid getting all the rear suspension geometry right with such an increase in track width is above my pay grade. Good luck and when you are finished, pics are mandatory
For those who don't remember, Ali is the guy who is doing a cut and stretch on a Boxster, dropping in a monster high displacement motor and then fitting some mystery (Bugatti???) kit car body around it. I am not this projects #1 fan but I appreciate his determination and hey... whatever floats your boat. The results are guaranteed to be interesting to say the least.