With no other mods, maybe 10-15 hp peak and a few extra ft.-lbs. over most of the powerband. Spend a lot of time reading williaty's posts on Nasioc in the NA tuning forum. And ignore basically everyone else except Matt Monson.
Cliff notes version is that you shouldn't do anything but tuning, a TWE header and cams, unless you also do pistons. Also plan to use 91 octane gas if you want to get tuned.