I've noticed this in a league race last night... I decided to do a late pitstop and changed my settings from R3's to R2's to be able to push in the last couple of laps.
When I came in for the pitstop it changed tyres, but in the end I also had to wait for "Setup Changes".

I think this is not realistic, because the tyres had already been mounted. I can understand when changing wing settings or camber settings lengthens the pitstop time, but in my opinion whether the imaginary pit crew mounts R3's, R2's or R4's, it shouldn't make a difference for pitstop duration.