Don't put your timing below 10 degrees. You will loose power and have idle problems, as far as I am aware. Advancing the timing will indeed give you horsepower. You must run premium gas though. Advance it to 14* and see if it pings. If it dosen't, you are good to go. Enjoy your free 5-7 hp.
I just set mine today at about 14. It started pinging a little bit at full throttle@4000 using Sunoco mid-grade. I put Sunoco 93 in it and the ping is gone. You can feel a slight difference in the pants. The idle went from 850 to 1100 so I turned it back down.
Those are accurate times for a stock 5.0. It also difers from motor cam combo. I run 19-22 deg with 39-41 total. My friend runs 36 locked out timing. Push it as far as you can, then find out what feels the best when you butt dyno it. Just as long as it doesn't detonate.
I run at 16* (granted, I don't have anywhere near a stock motor), but it tends to run quite well over the 14* I initially set it to when I built the motor.