You've upgraded to Brembo's right?
They look like someone's had a go at recon on them. Was it you? They are sided so need to go back together the right way. If they are put in the same box or something the sides can easily get mixed up and get reassembled wrong, as in, one half of the right side getting reassembled to one half of the left side.
As I'm not too clear on exactly what you have tried and not tried, this is what I advise and it is going to be the quickest way to get to the root of the problem, in minutes.
Here's what I would like you to do:
Conduct a quick inspection of the brake lines from the front to the rear in order to ensure that they have not been modified or capped by the Rally Team as per the former drivers request. Sometimes they make strange modifications......
If there are no such modifications in place then it's time to move on:
Check the inner and outer nipples are both facing the same way, as in they are opposite each other.
If they are, you now need to remove the brake line on one of the rear Calipers via the banjo and check the flow when the pedal is being pumped. If the brake fluid is flowing real good then pinch the line and then commence to remove the line via the banjo on the opposite side and repeat the exact same process on that side.
If both are flowing great then remove both calipers and bench them for the final part of the process, which is:
Remove both inner and outer nipples and then, using a pressurised airline, force air in through the inner nipple and see if it's either getting full on back pressure or if it's trying to exit through the outer nipple. At this stage, you will need to ensure that the piston is held in position so that the air has only one path to take.
If the galleries are clogged internally, hopefully the air pressure should help to ease any clogging within them.
Repeat the same process over again via the outer nipple.
There are only 3 possible scenarios here:
1. Brake line or lines are clogged or capped.
2. Caliper or Calipers are clogged.
3. They have been split / reassembled incorrectly. As they are sided.
It's an RS so there's no ABS system to blame.