Hey, c'mon guys, the subframe connectors and strut-tower brace add something like 30lbs of downforce! AND you can call it weight-reduction, too, from the cash-ectomy our of your wallet.
1) Driver mod first. You will see the biggest improvement here, in all areas.
2) Brakes: Doesn't matter how fast you go; if you can't stop... Minimum good pads and ducts. Think longevity and fade-resistance, and not bling.
3) NOTHING ELSE. Until you have developed #1 to the point where you can consistently identify where something about the car is holding you back, then fix that, and only that. Lather/rinse/repeat. Also, when I say "identify," I'm talking about specifics, not generalizations. "The car pushes (understeers)." Okay, fine. Does it do that on initial turn in? Corner entry? Mid-corner (around apex)? Corner exit? Each phase of the corner has different things going on in terms of vehicle dynamics, and different root causes to the example understeer problem. Also, the biggest benefit to doing it this way is that you learn to identify how the car "feels" in all the different situations, as well as learning to drive around those issues. It makes you a better, more rounded driver. Because it's a skill-set, and not a piece of hardware, you can then hop in anything and in just a few corners pretty well have the car figured out. You will be fast in whatever you're in.
