By all the reports, you probably won't need it on the 1250!
I may have misunderstood this, but from what I have read I think BMW introduced extra checks on the O2 sensors when the bike switches on and the fail when the EASY or AF-XIED units are fitted. I think someone was developing new hardware/firmware for the AF-XIED units so that the bike would see the O2 sensors during its checks but then control of these would be taken over by the AF-XIED unit after the checks were passed. Not sure what progress has been made on this.
What you are saying is all correct. However, I have yet to see an engine that does not benefit from a slightly richer than 14,7 AFR.
I have done a bit of research in this area as I built my own manipulator for my present -15 GSA (at a time when the AF-XIED did not work). I made it work, also after my bike got upgraded with the Euro 4 software. As such, I would therefor not need to buy a unit. Even so, DIY work is quite time consuming, so if a working unit is available for £150 it is tempting to save lots of work.
Like you say, the AF-XIED got a modification to make it possible to work on the Euro 4 engines. But in the usual American way (and no offence to our US friends), they are reluctant to modify a already working item. The first fix was a separate box with a relay that delays connection of the O2 sensor in the beginning (I've seen suggested 10 sec. delay), thus you needed 4 small boxes. Then they integrated a relay, so, now you are down to two boxes. Still, the Rapid unit handles two sensors, which is far more elegant. The only catch is, it seems not to be working on Euro 4.
As suggested, the 1250 is reported to be smooth, as may be said about the Euro 4 software for the latest 1200 LC engines as well.
As for my own -15 engine, it did pretty well prior to the Lambda manipulation as well. The manipulation did not offer a whiplash experience, but the engine felt more responsive when opening the throttle and generally ran even sweeter It is therefor very tempting to see what a manipulation will do to the 1250...