I don't know what the effective difference in braking is exactely between Cast & Billet but most very good racers at Club level don't go to the expense of billet, so I would not think this is very noticeable for us mere mortals!
There is no mounting for brake light switch on the GP Master Cylinders but this is easily corrected by using a pressure banjo that makes contact when there is pressure in the lines. Check at Demon Tweeks.
The standard reservoirs work with those although you might have to relocate them as they could get in the way with the fairing.
As far as upgrading the calipers, you would get ultimate performance with those, but let's face it, the normal 4 pads road set up + GP master cylinder is more than enough for our level of riding especially on the roads (well speaking for me here). Many guys use that set up with fully floatters in racing and seem to do very well with that. A popular choice is also the AP set up which seems like a bargain compared to the Brembo stuff.
I will be fitting soon some 32/36 to my SPS but this is mainly for fashion / pleasure of building a race rep as they are truly beautiful pieces of engineering. Well I did pick them up cheap too.
The thing though is to use their race discs too as they are slightly thicker than the 996r/998r ones and the pads being very narrow you need the narrow band discs to avoid using them unevenly (in case you ever want to revert to normal calipers... when selling the bike for instance).
Also you might get some great deals on those CNC second hand soon since racers using that kind of equipment should upgrade to radial mounts and those that have serious money should upgrade to the brand new 4 pads version as they use in GPs.
Check
www.redracingparts.com they are the best prices I have seen for them so far on the Net (but as KJ said, worth looking around for discounts).
All brembo Race calipers are 40mm or Radial. Only road calipers are 65mm mount.