Brett,
The OS 46 FXI was/is the engine that is sold with the Hobbico Nexstar RTF package. To my knowledge this is the only place that the FXI is used. You can't "buy" the FXI by itself. I had one for a bit and gave it away to two brothers that needed an engine. They use it every weekend and as far as I can see it runs and performs just like the 46 FX. I might be wrong, but I think that the "FXI" is just a name change done to make the Nexstar RTF package to "look" more impressive. It's just an OS 46 FX with a fancier name on it!!!!
Ken