I'm not familiar with Roe's product but you have to be careful that you don't end up with too much rear brake bias. If you are doubling the piston area (Roe says pad area but I presume he means piston area since increasing brake pad area does not increase clamping force) you are doubling the clamping force. Even if you leave the stock prop valve in place, this will probably still yield too much rear bias and early rear lockup. You might then have to place an adjustable prop valve to compensate.
I went with Tom Hayden's rear caliper upgrade which increases piston diameter from 36 to 40 mm. In combination with removing the prop valve o-ring, this appears to yield approximately the right balance although I have not had the car on the track yet with this upgrade. It's a simple bolt on as it's the stock caliper with enlarged pistons and priced fairly.
It seems to be an accepted fact that the stock brake system on the Viper has too little rear bias (I assume you don't have ABS) and any increase in rear brake bias can only help provided you maintain a proper balance.
