The brake clamping force on the rotor is proportional to the piston area. The total area of the pre-ABS Viper front pistons vs. the rear pistons is about 83% front/17% rear and the 40mm changes it to 80/20%. ABS Gen 2 cars are about 78/22%, and they can tolerate too-large rear brake area because the ABS will keep the rears from locking. These three examples kind of tell you where the sweet spot is.
Putting fronts on the rear and getting 50/50% is not going to balance the brakes! After you modify the knuckle, mount a replacement parking brake, use different rotors (or spacers) to get the front caliper to fit behind the rear wheel, install a proportioning valve to dial down the rear brake line pressure, you will use them at far less capacity than what they are capable of. I'm sure it gets great results, but you've spent a bit of time, money, and made some irreversible changes to get there.
If the car already has ABS, then it'll make the four piston caliper work as hard as, say, a one piston caliper. If it did any more, it'll lock the rears.
The OEM rear calipers are not particularly good looking, but they don't have to be much larger to make a big difference. The heat soak of a four piston aluminum caliper will transfer lots of heat to the fluid, while the cast steel single piston will not transfer as much, but either way boiling fluid in the rear isn't an issue. The total weight of brackets+caliper+parking brake is probably close. The only negative is that the smaller pads might run hotter, but I've yet to hear anyone complain they wear out rear pads faster than fronts. Reports from track rats with 40mm rears say it goes from 3 front sets + 1 rear set to 2.5 front sets + 1 set rear.
I posted deceleration data on the board after measuring the braking improvement from about 0.8 g's to over 1.0 g's just from the rear brake swap (stock '94, street tires, two people in the car.) It's a great post, because someone added SRT braking, which did better yet, but the SRT has ABS and better suspension geometry, too.
I think that the four piston calipers on the back look terrific, I am sure ABS would make even me look good, but I'd love to see the data that says just swapping in larger calipers (other than for endurance cars where you need huge pad area so they last a longer time) stop the car any better. And to be totally anal and technical, if your testimonial is that bigger calipers feel better, include the size of your master cylinder, since the ratio of master cylinder diameter to piston area will affect your modulation capability, so it feels under control, but may or may not stop quicker.
In God we trust, all others bring data!
