Ummm... I can tell all from experience that what kcobean said about designing brake systems is true, but it didn't happen on the Viper.
Federal safety standards require that every vehicle have a proportioning valve in order to prevent rear wheels locking up before the front. Automotive designs are focused on hitting the obstruction head-on, so seat belts, airbags, bumper design, crush zone, etc are all tuned for hitting the thing you see in front. Locking the rear first might easily swing the car around, and you would hit side- or rear-first. The average driver does not deal well with oversteer, the occupant protection is not as great, although with side air bags you can see that is changing.
The proof that the rear brakes are inadequate is that one can disable the proportioning valve and the fronts still lock first.
This is a completely different situation than "installing an adjustable proportioning valve." The reason to add a valve is to reduce the rear brake capability (i.e. when the car has 4-piston calipers in the back.) In the stock Viper, there is already not enough brake, so you don't ever want to throttle it down, you want to somehow increase the capability. The 40mm calipers increase brake clamping force by 23% and you still can disable the proportioning valve and not lock the fronts.
I'd love to know what pads with the stock rear calipers outdid the ones in the front. I tried and could never find that combination.
So to come back to the real world, since the stock rear calipers are relatively under-performing, using some kind of more aggressive rear pad is bound to be beneficial. Because of the original (poor) design, there is such a huge margin of safety or error, depending on how you look at it, on selecting front and rear pad combinations that it's very hard to see making a mistake.
