Stock my car was 430/420 at the wheels. I got the evolution stage 2 ( jlt intake, tune, 2.6 pulley, plugs ) 521/519 at the wheels now. I also have Bassani race mufflers but there is no gain with axlebacks really.
If you want stupid power get a whipple 3.4 or kenne bell. The ford racing tvs blower has a lot of potential as well.
Yo I'd go with the rest of these guys and get the Whipple 3.4 or the Kenne Bell 3.8, I know that Corza makes an exhaust exclusively for the 500 that's supposed to be nasty. I also think that you might want to look into getting a tune done by SCT and definately get the new JLT intake and tune deal from them. Or just the intake. Either way, the tune should be the last thing on the list because it should have to adjust to all your other mods power and really harness it even more... All these mods equal real raw power.
My car made 548RWHP on a Mustang Dyno with nothing more than a 2.6 pulley, a canned EVO/Lund tune, and the JLT intake. It runs consistent mid 11's on the stock radials, (stock gears, exhaust, suspension) and has been a best of 11.47@122. Best MPH is 124.5
The EVO or VMP "stage 2" kit would be a great starting point and will really wake the car up. My car stock seemed really slow to me, but now it is a completely different animal. Can't wait to see how it feels with the 3.4!
Mine put down 438 RWHP / 444 RWTQ stock. After JLT intake & custom tune by Justin @ VMP it hit 525 RWHP / 500 RWTQ. Added 87 RWHP & 56 RWTQ...not bad for about $850.