What should the values in useroptions.ini be for best quality?

Find the SweetFX_Settings.txt file in the main folder, you can change settings from there, also helps to read the instructions file for the hotkeys and stuff.

Personally like i said before i use Lumasharpen at a value of 2.0 and Vibrance and Curves at their default settings, everything else is off, but you could always experiment to find the settings you like best.
But it definitely should Not make the game darker unless there is some strange graphical conflict going on.

I know from experience that the other settings Do change stuff, Shadows at 4 changes their quality and most notably their quantity, they appear on a whole lot more objects.

Lighting at 4 also makes a difference in quality and number of effects you get.
They all make a difference in my experience but you can always go ahead and test them yourself one by one with comparison screenshots, i could be wrong but i doubt it since i notice changes.

Yesterday CyclesMchurtz mentioned that you can improve quality of the image by setting renderquality to 1.4 - someone else mentioned that you can set it even higher at 2.000000000 (approx num of noughts)

I tried it and the game looks very good. The image is crystal with very little sign of aliasing or texture shimmering. Takes a big hit on your system though. At 2.00000000 my 670 could only manage 25 - 30 in the Warpgate. I doubt even a 690 could handle it smoothly.

On my home PC I was able to disable depth of field blur (your rifle blurs when zoomed in, things at long distance blurs) but on the PC I'm using now I can't remember which option it was to disable it.

At 2x supersampling (1.41 renderquality) I get about 60~70 FPS in the warpgate GPU limited @ 80~90% GPU usage. I haven't tried the 2.00 setting yet, and agree that even my 690 would have trouble with it.

Also, to the OP: High lighting + High Shadows + fog shadows = a bad time, don't do it. Set shadows to medium instead, or turn off fog shadows.