Normally FPS loss is about none, wich is quite incredible considering the quality of the antialiasing they provide, especially SMAA.
Strange you're experiencing noticeable FPS loss, it runs so well on my "reasonnably powerful" rig; I assumed It could works pretty flawlessly on any PC.
About the results you get I'm sorry they didn't pleased you enough, but again, sweetFX without tweaking comes with very neutral presets.
So, not a surprise if you don't see a clear improvement.
I'll post screenshots as soon as I'll get my hand on COH2 and figured out some fine tweaks
Maybe it wont need them, but I've yet to find a game that can't be improved in some way or another, at least according to personnal tastes.
I'm not very worried about the legal outcome of using them; again, nobody as far as I know has ever been banned nor repeatedly kicked of a game because he used injectors.
I've been lead to believe that they do not act like cheats for the game's code, wich they don't interfere with anyway, and that anticheat company are well aware of that.
COH2 will probably be my top game for the next years, so I just wanted to sort this out 100% sure with the competent authorithies I've seen hang around here lately.
PS: and btw I've been using FXAA for years in COH1, so no risk at all with him.