Snow tires at 45-50 psi cold have stiff enough sidewalls.
And trust me on this... don't rally your RS. Don't rally any car you actually care about, and damn sure don't use one that's not paid for! I used to plan on running my RS in CRS at some point, until I realized that I would spend $10-15k in prep, in order to destroy my car in less than a season. Get a beater. Better yet, get a beater that's already prepped. I guarantee you can find a used rally car in close to race-ready condition for $5-7k. Every year.