We would suggest not to upgrade the radiator. From our experience, the radiator from BMW is more than adequate to do the job.
We're still using the stock radiator from BMW on our 258rwhp 318ti. From our data logs of circuit racing, coolant temps started off on 92C/198F degrees and increased to 97C/207F degrees on the warm up lap. Once we started to give some stick and get some airflow through the radiator, temps dropped down to 85C/185F degrees.
Normal daily traffic temps can get as high as 105C/221F degrees.
At the end of the day, it's your decision to upgrade the radiator or not. I hope this data helps to give you some insight