No personal experience just a general opinion from what I've read on some German boards while searching for the big-bore options.
There are a few BMW tuners that use the Rexxer for their projects and they seem to prefer this above external units because there will be no latency as it is programmed right into the ECU.
My fear is that when you would visit the BMW dealership for maintenance and they decide to update your ECU with new firmware the program will get overwritten and then you will need to revisit the tuner company unless you buy the Rexxer unit and manage to get the tuning map onto it. I'm not sure one of the default or pro maps would be sufficient for you, my guess is you want one custom made to your bike
So maybe your best bet would be to inform a tuner