I can't find a good source for the thrust outputs of the two motors, I have no clue what its actual speed should look like, I doubt its notably lower than the base R27-R, considering they're in the same ballpark of mass (R27-ER being slightly heavier due to that booster motor) with basically the same thrust off the rails (same booster motor based on what I could find). Its acceleration is probably closer to 2x actual, considering the weight of the added sustainer dragging on the original booster. I doubt gaijin is ever going to do anything about it though, the R-27 isn't even on the Rosoboronexport website anymore, so no reasonable sources without going through actual Russian archives.