To be honest, it's a finite system, so with the parts you have and what I mentioned, there's little left on the table besides turbo and internal engine mods.
Maximize air in: bigger, less restrictive hoses and pipes, intake, spacers
Minimize temperatures: intercooler, radiator, oil cooler, intake
Maximize air out: catless downpipe, high flow exhaust
Maximize turbo efficiency: BOV/IWG, manifold, inlet elbow, turbo core
Decrease rotational mass: lightweight wheels, driveshaft, flywheel
Maximize engine potential: cams, forged pistons/rods, custom ecu mapping, good gas
And remember, power without traction is useless. Good tires and subframe bracing help, so does an lsd.
ETA: What the other members posted doesn't increase power, however it increases driveability which translates to quicker times, which is the end goal anyway, right? Transmission plate, aluminum bushings and brakes definitely won't hurt.
You can also throw in "insurance" mods like an oil catch can and crankcase breather to give you peace of mind.