Same thoughts here. On non-turbo cars I always tested this by revving out 1st, putting it in 4th, and flooring it at a low RPM. If it started slipping it was usually a sign the clutch was worn out. I would advise you don't do that with the RS due to LSPI and all.My first thought is clutch slip as well. When the clutch is starting to slip, it will typically slip in the higher gears where there is more load, like what you are experiencing now.
The way I test turbo cars is get a nice, smooth, straight road, get into 3rd and at a safe RPM (2500-3000), just floor it. If the car consistently pulls and no funny RPM jumps your clutch is likely okay.