The R has an oil cooler and - if the car has DSG transmission - a DSG oil cooler and they’re located behind the two outer grilles in the front bumper. The GTI’s front bumper doesn’t have those additional grilles so wouldn’t provide the necessary airflow to those two auxiliary coolers. Therefore fitting a GTI bumper would likely result in overheating issues.
For that reason, I wouldn’t change the original R’s front bumper for one from a GTI.