I've actually been digging into this in my limited free time to add ACC to my 22 RST. IMO, it seems attainable, at what cost, well that's part of what I'm working out.
I haven't been able to confirm the presence of the wiring harness' for what needs added, but like WalleyeMike said, you need to add the "Long Distance Radar Module" and bracket (behind front bumper/fascia on passenger side mounted to lower radiator brace) and calibrate, along with the "Image Processing Module" plus bracket and calibrate (located behind the last trim panel on the passenger side rear "wall" against the hatch, mounted beneath one of the SRS modules).
Then you'll more than likely have to calibrate the DIC to accommodate adding ACC. The standard switch on the wheel seems to be the same as the switch used for ACC control.
In my case, I already have a front facing camera module and sensor assembly.
My largest concern (besides the cost of that IPM module, it's a spicy one) is with the calibrations themselves. I'm using a VIN from a High Country to do my research and eventual upgrading/calibrating. The question I have now, is if GM "codes" the calibration files to the VIN used to find the calibration you want. If they do, that's where a guy is going to have to get crafty.
My RST is currently in the shop for some warranty work but I do plan on trying some things when I get it back.
Funny comparison though, on the flip side of manufacturers, with Ford and their Super Duty line you can just add the long range radar sensor, calibrate a couple modules with a 3rd party Ford program called Forscan, and it'll work.