Current mods in sig. Retuning with water methanol injection.

Currently 20psi, how much boost do you think I get out of the turbo? (precision 6031) is 25, 28, 30 psi possible? My car is a weekend warrior, and is beat on everytime its driven. I want to get the most out of my setup, and frankly I am not worried about my pistons since peeps have made 500whp+ on them, and I know I won't see that much power with my set up. I guess the weak link in my set up is my turbo.

Also, with 500 miles on my new pistons, my pistons are brown and ugly looking, what is the cause of this? Running rich? 90% of the time my afrs are 10-12, and all sorts of throttle positions.

Any info you can supply would be great.