the rear sway is a great mod. I'd definitely recommend it. The front is good if you like crisp handling and are not trying strictly to remove understeer (which it will actually increase)
Endlinks don't really do much in terms of capabilities but they do give clearer feedback to the driver about what the sways are doing and better feedback makes for better driving. I would caution though that metal on metal endlinks are the best performance wise, but they can be rather noisy up front. It's fine for the rear but if you mind the clank of an endlink when you take a turn, I'd either leave the front endlinks alone or get ones that still use some measure of padding.
Lowering springs are purely for looks. They do not help your handling at all. In fact, some tests have shown they can substantially reduce the handling capabilities of the car.
Coilovers are completely overkill for a non-track driven car. Their strength is NOT in improving handling, it is in ADJUSTABILITY. If you're not going to be adjusting them, they're a total waste.
But then again, people love spending money on "performance" parts for their car that do nothing. How many spoilers do you see out there that are actually functional? If you want your car to look "tuned" or something, get some cheap ass lowering springs. Just don't tell your buddies it's helping anything or one of them might know enough to make you look stupid.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD