I shot about 400 rounds through the P09 and dry practiced it to death before making any mods so I could really tell the difference (if any) with the mods that I planned to put in.
The gold hammer spring from CGW helps lighten the DA pull . I didn't setup an extended firing pin or anything and the gun has ran fine with GECO, PMC, Fiocchi, and American Eagle. I bought up another trigger sear too, and polished it up and it helped with the grit in the single action pull. You can feel the stagey trigger through the double action pull even with the lights hammer spring, but it isn't nearly as significant. Also with how fast you are normally running a trigger you don't feel the grit in the SA pull.
Polishing the sear, and the gold hammer spring are not must haves but definitely recommended.
As far as sights, I took the front sight off and used fluorescent orange on it and blacked out the rear sights. Made it much easier to pick up.
