Unfortunately staying 4-lug limits you very badly.
For ~$200 many guys have upgraded to Sn95 style spindles and hub and run the 99-04 GT 2-piston calipers. Better clamping force, better style rotor/hub setup allowing more efficient cooling, more modern brake setup. But you would have to go 5-lug.
SS lines are a great deal. I won't knock them at all.
As for rotors, drilling holes and slots is not an upgrade on a rotor already prone to "warpage" due to heat. MASS is what cools a rotor, not fancy holes and slots perpendicular to the airflow out of the rotor. The Mustang already suffers from undersized brakes coupled to a integrated hub...it doesn't need to be crippled more by reducing the mass of the rotor and concentrating the heat into a lesser area of mass. Warpage is friction material transfer from the pads to the rotors unevenly by excess heat. If you want to reduce tendency to "warp", you gotta keep the brakes cool by way of air ducting, solid plain-faced rotors and pads that can take higher heat.
The problem isn't what you get in the parts you listed. They all do offer some improvement. If the kit costs $100 or so i'd say go for it. It's just that the cost associated with making a stock brake system decent can be excessive for marginal gains.
I'm not talking out of my ass here. I've run pretty much every fox brake setup you can think of.
Stock 4-lug
4-lug with 73mm calipers
5-lug with 73mm calipers
5-lug SVO front and rear
5-lug SN95 GT front and rear
and now Cobra 13" front, 12" rear.
Toss in SS lines, hawk pads, MM caliper bushings, fancy rotors, you name it on all those setups. I wasted a lot of money, and in the end, my $700 5-lug Cobra setup was the best deal I should have done from the beginning.
