Good job.
I also prefer performance modifications over looks/style, but I'm a motorsport engineer and it's really just about whatever makes you happy.
If it were me I'd spend a bunch on the suspension/brakes and then throw together a twin turbo setup
Suspension/brake ftw. Twin Turbo set up, now THATs something that requires a fat wallet! I can't believe the numbers the V6's pump out on the '05 Mustang and up.
If I were the OP, I'd throw some cash down on some bigger drilled/slotted rotors and calipers. Then gas adjust shocks/heavy duty springs, if not adjustable coil overs all around, and sway bars. Just bolt on parts, it would handle like a dream.
For the engine, again just bolt on. Cold air intake, mass air flow censor, an aftermarket ecu if they make any for that motor (I don't really keep up on Ford), a dual exhuast system, flowmaster 40 series. Then if you had the money, more cam. The motor would breath a ton better. I'll stop there. I ramble way too easily, but this is coming from a guy who day dreams about automotive racing all day long. American Le Mans series ftw.
I know in california you can get ticketed for having an aftermarket air cleaner if it doesn't have the hippi approval stamp.
California is getting way, way, way out of hand with automotive regulations. I can't believe a law might get passed there that makes it so you can't purchase a new car that's painted black or a dark color because of how much harder the AC has to work on hot days compared to lighter colored cars. And up here in Oregon they're talking about making it illegal to add anything to your car that would hurt gas mileage, like a bike rack, etc.