There is no need to go to a lager bore master cylinder. For one, doing so will lower the pressure output for the same given peddle effort and therefor you will have to push harder to equal the same M/C output pressure, secondly, the stock master cylinder will displace more then enough volume for a 6pot caliper setup. The BBK that I built on my car is using 6pot calipers and the peddle is actually stiffer then the stock peddle feel. Generally speeking, just cause you have more pistons, does not mean that you have more area or volume. Remember, as piston count goes up, the piston size goes down. The only reason for multiple pistons is to spread the clamping force more evenly across the pad and also it you notice, multi piston calipers that use long pads actually have different size pistons they start out small and get larger in the direction of rotor sweep (rotor rotational direction). This is to even out the clamping force. The uneven clamping force of even sized multi piston calipers with long pads can easily be seen on the older 4pot corvette calipers. They always had what is called taper pad wear. This is bacause the trailing side of the pad actually gets pulled into the the rotor by the rotor as it rotates through the caliper and this causes the leading side to get pushed out and therefor the trailing half of the pad would do more work and wear faster then the leading half. To overcome this, manufactures (like Wilwood, Brembo, AP Racing, Alcon, ect...) started putting smaller smaller pistons on the trailing half of the pad and or larger pistons on the leading half. The larger size leading pistons then could overcome the the pad being pushed out on the leading half and therefor even out the clamping force across the entire length of the pad.
Back to the Volume issue, most multi piston calipers if sized correctly for a given vehical will have very little differance if any at all in over all piston area.
My Wilwood 6pots have almost identical piston area as the stock MKIII single piston front calipers. Actually, IIRC they have a minut bit less area then the stock calipers, yet the braking is far better. Something else to remember is that clamping pressure is only a single part in the over all picture of braking. Brake torque is what we want to increase to increase the braking ability. There are three things that effect the over all torque, clamp pressure, pad to rotor friction, and rotor diameter are those three things. Rotor diameter and pad coefficient of friction and more of an effect on braking torque then clamping pressure. Increasing pad friction coefficient and rotor diameter will significantly increase braking torque. This is why most well known quality BBK companies such as StopTech, Brembo, AP Racing, ect... actually build there BBK's for any given vehical with equial or less caliper piston area and therefor less clamping force. This helps with peddle feel and keeps proper front to rear ballance. Although in a lot of instances, the rears can stand to have more bias then what the factories designed them to have depending on vehical setup.
One thing to always remember is that brakes do not stop your vehical, tires do. You can have the biggest, meanest brakes in the world and if your tires are shit, the car is not going to stop any better then it did with stock brakes. The main reason behind BBK's is heat managment. Auto manufacturs spend a lot of time designing the brake system to stop a particular car and the best that they can for the given setup, tire size, and aver tire traction but the problem is that most of the factory designed systems are not designed to handle repeated hard stops and slowdowns from high speeds like say a raod course racecar does. Doing so and you'll likely experiance brake fade and warp and crack rotors. This is where BBK's come into play. Larger thicker rotors will absorb and dissipate heat better then smaller thinner rotors are therefor far less likely to experiance brake fade and warping and cracking of the rotors.
