The factory Rotax FADEC is very sophisticated and therefore expensive, given the development and testing behind it. I am not sure if they run LOP.
In our testing with the Bing carbs, they were very well matched to the engine. The average AFRs (measured with a wideband AFR meter) were quite precise in almost all ranges, a little rich around 4500 if I recall. I was frankly surprised at how well they worked. The bigger problem was cylinder to cylinder mixture variation, clearly two cylinders run richer than the other two from EGT and plug color. With EFI, the EGT spread was dramatically less.
At WOT, the carbs showed a slight pressure drop. The larger EFI throttle plate we used and plenum manifold showed no loss in MAP at 5800 rpm so between these two things, the engine picked up 3-5 hp (80 hp test engine). Further independent testing showed fuel flows were about the same amount lower at the same hp. So the gains were small but useful with our system.
In testing the factory ignition, again Rotax engineers got it right. We saw no gains in hp using our programmable ignition however at higher altitudes, there would likely be some gains by advancing timing.
You can see some photos of the testing here:
http://www.sdsefi.com/rotax2.htm