No surprise you like this car, it's fast (3 mph faster on the top end), light (feeling), twitchy (like it's an E36), and just very loose feeling. The car feels like it's wearing 225 series tires when the RSR is wearing 325's. No doubt it's faster in your hands, but being much less confidence inspiring it was slower for me. It also suffers from what a lot of GTS cars suffer from (not exactly sure why), which is initial throttle instability, so I had to use TC3, whereas I use TC2 with the RSR.
My changes to all these stock GR3 cars are the same.
All lightness
All power
TC2
Racing Soft tires
All downforce
Diff @ 20/5/20
Trans fully custom with just the top speed changed to 193 mph
I do a 6:21 in the RSR, 6:26 in the M3. I'm betting with the same settings you'd do a 6:14. You should try it.