I did a lot, probably too much, reading and investigation on the Booster plug and the AF-Xied.
If you have a spare afternoon, follow the rabbit hole that is Roger 04 RT, the developer of the AF- Xied.
https://www.ukgser.com/forums/showthread.php/482858-AF-XIED-for-R1200-LCBasically, the Booster plug fools the ecu initially in thinking it is colder than it is, and therefore dumps more fuel in. It plugs into the temperature sensor feed. The BMW ecu, using the other sensors including the o2 sensor, adjusts for the booster plug and it reverts to standard.
The Af-xied adjusts the o2 sensors reading to increase fuelling in closed-loop operation, and by learning, open loop operation after a few tanks of petrol. It is also adjustable depending on what aftermarket goodies, e.g. exhaust of air filter you have installed.
I've installed the AF-xied on a couple of bikes, including my current one and the result is noticeable and sustainable. But don't take my word for it, have a read of the link and search for all the discussions and data on it.