Anyway, to get back on track, does anyone know how how mixing and matching brake parts would affect performance? Instead of my XT/SLX example, what about pairing a Deore brake lever with a Saint caliper, and a Saint lever with a Deore caliper? How would the performance from each of those setups compare to stock Saint and Doeore brakes?

Below is one way to approach braking power.

The master cylinders convert your applied braking force to pressure. The calipers convert pressure back to force. The hoses allow the fluid to flow in order to transmit pressure. We can assume the viscous energy losses in the hoses are negligible.

Therefore the operative mechanism is this: When you pull the lever, a piston converts that force into pressure. The pressure is related to the force based on the leverage ratio of the lever hinge and the width of the master cylinder piston. Pressure varies with force according to P=F/A where P is pressure, F is force, and A is area of the piston =pi*radius^2. This relation is true in both the master cylinder and in the caliper.

To put it all together:

You put a force F on the lever at some distance D from the pivot. The resulting force on the master cylinder is F*D/d, where d is the distance from the pivot to the master cylinder piston pushrod. The master cylinder has a piston diameter Dp. The caliper has a pistion diameter dp. The pads apply drag on the rotor according to some constant Uk. Thus the drag force on the rotor (F_rotor) is

F_rotor=F*D/d*dp/Dp*Uk

To convert this to Power (P), consider P=energy/time = force*distance/time. To calculate braking power, take F_rotor*s/t where s is 2*pi*r where r is rotor diameter and t is time you have been applying brakes.

To get fancy, work everything as integrals and consider brake fade where Uk is a function of T (temperature) and T is a function of power and airflow and conduction, resulting in coupled equations. Within that power function can be convective and conductive heat transfer effects and pretty soon you need a Nusselt number and things start getting interesting (see:

Nusselt number - Wikipedia, the free encyclopedia )

What this all means is you have to consider the ratio of area of the master cylinder to the area of the caliper pistons. For a given applied lever force, the braking power scales inversely with the area of the master cylinder piston and linearly with the area of the caliper piston. So Saint calipers, which may have more caliper piston area than some other caliper, will have more braking power. To find how much more power is presented, you need to measure the total piston area.

That is, to a first approximation, the braking power scales with dp/Dp and you need to find the ratio of those dimensions to compare various setups. Lever excursion will vary with the inverse.