Maybe the FK2R rims width are 8.5j while the RS275 are 8j?
Maybe Honda and Renault want these cars to be able to use the best performance tires out there so they can outrun each other, and therefore they just opt for the 19'. Coz CSC6 and PSC2 are only available 19' up.
good insight, yes they are in fact 19x8.5, just found out the Speedlline Turini wheels on the Trophy are in not so common 19X8.25 even their 18" wheels on the regular Megane RS are in 18x8.25
I haven't done much research on the CSC6 to be honest, now that you mentioned how they are only available in 19" and up it deserves a good read