I have a Caliber R/T manual transmission that I campaign in the 24 Hours of Lemons endurance road race:
Dodge Caliber Race Car - 24 Hours of Lemons Road Race Recap!
When we were first setting up the car for racing, we looked at going with the SRT brakes, but we had other things to focus on, so we kept the stock brakes. We put on plain (not slotted or drilled) EBC rotors and we had custom brake pads made by Porterfield: ST-43 pads in front and R4 in back. We used Motul RBF600 brake fluid and we used a code scanner to help flush out the ABS pump. Porterfield also custom made braided stainless steel brake lines for the car.
The ST-43 pads are a bit noisy, okay for a dedicated track car, but if you will also use your car on the street then I would use Porterfield R4S, or maybe R4 if you are really pushing it, pads all around. Also, while they have to custom make the ST-43 pads for the Caliber, I think they stock R4 and R4S pads for our cars.
We've run over 2,000 laps flat out with that braking setup (we've changed pads twice, but are still on the original rotors). We push the brakes very hard every lap, braking from 100+ mph down to about 40 for the hairpins at the end of the long straights, and we run full 8 hours sessions at this tempo. My team is in full agreement that we have plenty of braking power and that there is no need to upgrade our brakes further.
Given that you will not push your car nearly as hard as that in autocross, I'm confident you do not need to upgrade to SRT brakes. I think it would actually be a detriment because they will be heavier but are not needed.
Instead of converting your rear brakes, I would call Porterfield and ask them if they will put the R4S pad material on drum shoes to fit your Caliber. I am guessing they will do it and that will give you more than enough braking power.
Porterfield:
2007/DODGE/CALIBER R/T - 2.4L/Porterfield Brakes - Porterfield Brakes
If you are set on converting your rear brakes to discs you should go to a parts web site like dodgeparts.com and study the brake and rear suspension assembly differences for Calibers with rear discs and rear drums. Note all the parts that are different and then go get them at a junkyard. You should probably replace the rear wheel bearings while you are in there.
More important than converting to rear disc brakes, if you are going to autocross, take a look at these suspension updates:
Do the subframes have bushings where they interface with...
