I have been testing this for the past 4 races and I see a few wee details missing or not right in the viewer and race data.
I currently have one car going into Monaco with roughly a 54% engine (same engine used since Australia) and the other with 100% (new engine) to get screen-shots, but even without them, what I have found is there is no difference in Top speed or Acceleration in the race viewer. The car is just slower without showing any visual reasons as to why it is slower. Even the after race data the average speeds are the same as everyone else.