According to PHP, JBA headers and an axle-back muffler gave them 17 RWHP. Adding a retune, and air intake raised it up to 20 RWHP gain. Their V6 started at 170 RWHP and went up to 190.
http://www.jbaheaders.com/articles/50Mustang_306.pdf
Add a pulley and a higher octane tune and you'll...