Where are the guidelines for rtr? I might head to Lexington ky Sunday and try that class
RTR is the same as stock class (FS), only on street tires (if I remember right, the minimal rating of 140). The cars from different stock classes get indexed according to their original class'es performance. And no, it's not a seriously competitive car in either FS or RTR
I actually ran my GXP at a National tour event in 2012, got the last place in class. I've done no clean run in the 1st day, but even if my best run were clean, I still would have been last after two days by about 0.01 seconds

Maybe the Yokohama tires were not the best choice.