Not recommended - it's built for the turbo, and will have a vastly different fuel delivery band. If you were turbocharging your RS, even then it would be sketchy, as the displacement would be different, causing a lean fuel condition (unless you mod it) and lean = detonation with a turbo, which will destroy your engine very fast.
There are several reason why it would not be a good idea, on top of what James mentioned. The WRX is a MAF based car, and depending on what year RS you have, you may run into some problems resulting from having a MAP sensor. I'm sure it could be done, though installing something like a S-AFC to add/take away more fuel once the ECU was in probably wouldn't be a bad idea. The RS and WRX have two VERY different compression ratio's, and timing would be another factor. But, if your RS is turbo then the retarded timing and increased fuel might be a plus.
I would be very intersted to see how a WRX ECU ran a turbo RS, hmmm.
It would be a 2002 RS. That has a MAF senser somewhere on it. I was just wondering if it would work. I am not planning on doing it. When I do put a turbo on there it will be one I brought from somewhere else. Unless I put a supercharger on it.