1) Yes. Framerate is going to be a bit different between mods and it all depends on how powerful your computer is. EfxMod provides a lot of shader additions to make things look a bit more like FPSCX10, however it causes massive slow down on low-midend computers. Enemy AI is all a matter of perspective. You can script yourself some "better" AI, though. The current Mods (To my recollection) are: efxMod, AirMod, Ply's Mod, and S4Real Mod. You may wanna search those up and do a bit of research on each to find which one would suit you best.
Keep in mind, all Mods, as of the time I am writing this, are reliant on FPSC V1.07 and NOT the latest update. The modders need to have the latest source code in order to Mod it, so you'll have to wait until then for the Mods to get everything that the latest updates have.
2) Do a small search around the forums for that one. There are many different approaches you can take for that.
3) Yeah... search the showcase board for some.
The one and only,
Whosoever says, "Don't sweat the small stuff," is obviously not a programmer.