Had a friend that dynoed his stock 03 and it put down 373 on the dyno which would put his HP at the motor at 428 w/ a 15% drive train loss. Much more then Fords advertised 390
There's a guy around here with a ported blower, pulley(don't know the size), full exhaust, and tune that put down right around 500rwhp with similar torque numbers. I know he ran around 12.20-12.40ish at the track when it wasn't prepped at all. I don't know if he's ran it since then. He spun a rod bearing last year and just got the car running again a month or two ago.
Most of them max out around 475-525rwhp with the stock blower. With a kenne bell they're good for something like 600+.
Guy i work with used to own an '04 CObra with 510 rwhp.
And he autocrossed with it! It was honestly the best handling Mustang i've ever ridden in. You would never think a stang could turn the way his did. He was also a very good driver.
he would routinely take Fastest time of the day on the autocross course......with a 500+RWHP Mustang.
But he sold it and bought a supercharged Miata to autocross with now