What settings are you using?
I don't use SweetFX, but I do Color/Gamma correct my screenshots in Photoshop. Couldn't tell you exact values as I do every single one on a case by case basis.
More work, but I prefer the results. I far prefer moving sliders and immediately seeing the impact on the image over inputting numerical values into a Notepad file and Alt+Tab'ing back to the game.
After I'm done I save the corrected image and compare it to the original by rapidly switching back and forth between the two for about 30 seconds in Windows Photo Viewer to make sure that I like the changes and didn't "ruin" the original.
It's usually super subtle, though, like "Do I like -5 Brightness +10 Contrast or +5 Brightness +25 Contrast better?".