Welcome, and lookin' very clean!
Given the mods on your RS; there's a high probability it was, or still is tuned. From your post; I assume you didn't get a tuner (i.e. Cobb AP, etc) with your RS. Doesn't mean there ISN'T a tune in it already. If I were you; I'd burn the highest octane pump gas (e.g. 93 octane) I could get as a protective measure; in case there is a tune in your PCM/ECU (this happened to me with my RS; came with a CAI, catless DP, and a catback exhaust--and unknown to me a tune of unknown parentage [didn't get the tuner that should have came with the RS]). "If" there is a tune in you RS; you can burn a lesser grade gas than what the tune is specced for, but eventually that could harm your engine. BE ADVISED!
BTW: An OEM tune only requires 91 octane.
Catless DP: Depends on which state you're in; and how assertive your state environmental regulatory agency is. I'm in MI. Our Peoples Democratic Party Regime is dead square on tuning shops ***es over Federal EPA Regs. Getting caught violating Federal EPA regs (e.g;. tuning a non EPA compliant car will likely cost them their State License/Certification--so most won't touch a non EPA car--happened to me--though the generousaity; of Snox801, I replaced my catless DP with his very low mileage OEM catted DP).
You need to get your RS into a competent shop (NOT my dealership; who's PCM/ECU Tech couldn't tell OEM from aftermarket code) to determine what kind of code you've got in your PCM/ECU. If there's a tune in there; whatever brand tuner the previous owner used "may" have installed what's known as a Strategy Code (software firewall that prevents an easy reflashing of your PCM/ECU to another standard [e.g.; OEM or a different tune]).
Unlike the smarter members of the forum; I've been the TEST DUMMY for all this ****, so feel free to post further here or PM me when you can.
Best wishes on getting your beauty of an RS sorted to your liking!