I put C discs on an auto x speedster, S vented brakes on a pumped 914 and F-50 calipers and 355x32 fronts and 330x28 rear discs with Lotus calipers and brembo parking brakes on a 308 gtb. I love brakes. Driving hard, you gotta have the ability to get rid of the heat or you are going to pay for a lot of rotors, pads and caliper rebuilds. It is imperative to stay out of the eating up the brakes zone.
IMHO, improving the brakes is a lot cheaper than paying for more HP in reducing lap times + Better brakes makes you feel almost invinceable - I hate having to back off early and baby really hot brakes.
So you want to have "just enough" brake heat dissipating ability to stay out of the "non-linear" Zone.
Assuming you have the air ducting close and once you get the diameter as big as you can front and rear, have a good F/R balance, (just not locking the rears), probably need a cockpit adj prop valve for this, THEN you can start reducing the disc mass/experimenting with pad and disc material, (titanium, aluminum, CF, unobtanium).
You can always substitute $$$$ for mass.
I went WAY overkill with the 308 giant brakes - i was dealing with ebay used parts and trying to get a close F/R balance, having hats and caliper adaptors made and used parts, but i still spent 7 k with me doing all of the install and brake line fab stuff. Martin at Girodisc was instrumental in making this happen.
Even with these huge calipers and discs, i have less total brake component "weight" than the stock ATE brakes, (identical to early 911 vented brakes).
But, i have huge flywheels at the front and probably a lot more disc than i need at the rear - if i trip over a huge barrel of money, i will start downsizing the mass and maybe the material, (but not the diameter) of the discs. I think, once you have the gross size of the disc set up, you might be able to go with a lighter version of the 355x32 disc, or, even a different material without irreparably changing the brake balance. You might need to do the same front and rear, so the thermal throughput remains the same. This is even more complex than i simplistically thought. Ah well, yet another shortcoming.
chris
As we speak, (or rather i blather on), i am rebuilding the brakes and suspension on my dad's 74 911, (including zinc plating the spring plates and calipers, new rotors, stainless lines and doing the elephant suspension bushings, late adj spring plates and bilsteins, hopefully being able to borrow scales to corner balance. Dunlop direzzionales on the stock refinished Fuchs, Carrera bars...TIGHT.