Hi all, I'll summarize, hopefully being fair and factual, and somewhat historical. Please correct any comments that seem overly or subtly self serving.
Stoptech's front kit uses smaller pistons (36/40 instead of 38/42) to shift bias to the rear. The first rear upgrade effort was to match their brake clamping force distribution. Mathematically this is done with a 38mm rear combined with OEM fronts and it provides the same balance as a 36mm (OEM) rear with Stoptech fronts.
The casting of the OEM rear caliper can tolerate a still larger piston, so the 40mm was born. It's still not too large and does not need an adjustable proportioning valve. Evidence that the Stoptech front only caliper setup is still front-biased comes from Chuck 98 RT/10 who has Stoptech fronts and then added 38mm calipers in the rear. Mathematically, he now has the same balance as OEM fronts and 40mm rears.
For everyday driving, autocross, and track day events (pad change, fluid change) the 40mm rear is plug and play. It's a few hour job, doesn't require cutting off the rear upright brake arms, uses all the OEM parts, and maintains the parking brake.
Continuing up the performance ladder is the four-wheel four piston caliper kit. It's not on the chart above because the rear bias would calculate to a huge amount. The math would point to any four piston rear caliper being about three pistons too large, and indeed an adjustable proportioning valve has to be added to dial down the clamping force and get back to the 20% rear bias range.
Why then have a large brake in the back? The component generating heat is the pad (against the rotor.) So that the heat isn't concentrated in a small area like the OEM pad (which makes it more difficult to maintain consistent friction performance since the temperature excursions will be larger) a larger pad is desirable. To have a larger pad, you need a larger caliper. The larger pad means you have more consistent brakes after the 100th hot lap, or some other very high heat energy transfer conditions. For the same reasons, a better cooling rotor is advantageous. In the rear there isn't the need for massive clamping force since a single piston caliper against the OEM rotor can always lock a wheel.
I've said (or tried to remember to say) if the braking levels are high enough to where heat transfer becomes the limiting factor, then using larger pads, larger diameter rotors, ducting or other forms of cooling become important (but remember we're talking about the back brakes, it's the fronts that will always generate much more heat.) Maybe another way of saying it is that the decision should be separated into 1) brake balance and 2) maximum heat transfer. That would bring up the details of what the braking system is really being asked to do.
Stoptech has some good technical papers on their site that have helped me. The URLs have changed a little since they were bought, so here is the current link.
http://www.stoptech.com/tech_info/tech_white_papers.shtml
