I'm not sure about the RS'es, but when I swapped a WRX dash into my L, quite a bit of trimming was involved. I used the WRX beam, which had to be cut and welded to fit right, the HVAC stuff was a pretty close fit, and the dash itself is fastened a lot differently. The two different firewalls are not even close for a bolt-in fit, unless my car was an exception. My dash is different from an RS'es, after all...
It's not impossible, just a whole lot of work, trial, and error.
But, once you get the fastening points set right, everything bolts in pretty close. The only plastic piece I had to cut was a little trimming on the console to get it to fit.
I don't know any right or wrong way to do it, but maybe there is a better way.