I hope to get my R9T soon and test it on my dyno. What I can say is that the original ECU on the latest BMW must not be confused with the ones from other bikes. Whereas on the most of them you have several injection tables, when it comes to the latest BMW, as for the cars, you find some o2 sensor target maps and torque request maps for the throttle. This means that the AFR will remain similar even changing the exhaust, as the injection will be adjusted to reach the same target.
Now, this target is not set to be "optimal". It is set to make the bike to pass the Euro3 and (from 2017) the Euro4 regulations... in other words, it's lean. Will this break the engine? No. But it will affect the throttle response and low-revs smoothness. I am not saying that the bike runs poorly, just saying that it could run better.
The actual AFR target for the closed loop (up to 5/6000 rpm) is 1, which means 14.7. Working on it you can improve the response of the bike, lower the temperatures and get a few HP, especially if paired with an exhaust and intake.
How to do so?
Well, there are many solutions. I personally work with Rapid Bike, they are injection units that adjust the AFR by using the original o2 sensor. This means that you do not need to get it mapped on a dyno, unless you are really looking for up to the last hp and you trust your tuner's experience and tools. The down side of these units is that you need to fit them and on such a bike it is not easy to hide it completely. I will let you know how I go with mine.
Another solution - probably the most popular - is to reflash the ECU. Nice clean job, but you cannot "erase" it for warranty purposes and the results will depend completely on the tuner's experience and tools.
I would personally avoid the booster plug. It makes the bike to run in a non-coherent condition (as if it was colder) improving a little area and damaging the rest. Since the "issue" is limited to the closed loop range, a cheap o2 modulator such as the Rapid Bike Easy is a way better option. Same concept (showing a different value to the ECU), but limited to the most problematic area, without interfering with important information such as the temperature of the air.
Finally, I would avoid the old generation of fuel units as they are not effective in the closed loop. To install them you need to blank the o2 sensor, in other words you blind the ECU of her most important fueling tool.