The cam journals are spaced the same in RR and non RR motors, so any DE cam will bolt in.
As someone said above, I think (not certain) the stock cams in the RR are larger than their non RR counterparts. Assuming this is true, you could bolt in S4s, BCs, or whatever; but they may not have the same gains as in the non RR motors. I'm also think (again not certain) that JWT made the S3Rs as big as they felt comfortable going, using the stock RR valvetrain. So if you found some larger non RR cams and put them in, you could be risking piston/valve contact.
You also have to consider that the profile (not just lift) of the cam is of some importance. It's kind of like the GTIR mechanical lifter cams in a regular DE. You can bolt in GTIR cams- it will work, and it will make more power, but it is reported to accelerate valvetrain wear.
I'm not an expert by any means, and I'm not saying these aren't options to be explored, just throwing some caution out there for consideration. Just because they fit and run, doesn't necessarily mean they are the best option. I know Snickers has said in the past that he could make some custom cams for RR motors. From my understanding, he is pretty knowledgeable with them. Perhaps he could provide a little more insight.