To extract more horsepower from your Subaru engine, you can use both hardware and software modifications. You will want to do your homework before you start throwing parts at the car, though -- badly-chosen modifications can do more harm than good.
In general, getting the most power from a turbo engine involves decreasing flow restrictions in and out of the engine, and adjusting the ECU programming. ECU changes include optimizing the air/fuel ratio, increasing turbo boost pressure, and adjusting ignition timing for the new fuel/boost settings.
The Cobb AccessPort is one of the first modifications for most of our customers. The AccessPort is an ECU reprogramming device that allows you to write revised engine control parameters to the car. It comes with several maps to start with, and more are available online. The AccessPort can also be used to enable custom tuning by a professional tuner.
Initial hardware modifications include exhaust upgrades (a cat-back exhaust, downpipe, or both), intake systems, and perhaps a gauge or two. Find out more about each of those parts in the Mild Modifications page.
Further down the rabbit hole are intake and exhaust mods that take a little more work/time/money to perform, including turbo inlet pipes, TGV deletes, and exhaust headers. These may or may not be done along with the installation of a larger turbocharger and upgraded fuel injectors, fuel pump, and intercooler. More details about these modifications are here at the Medium-Level Modifications page.
Ethanol (E85) fuel is a popular alternative fuel that can deliver big power gains, although there are trade-offs involved. Check out our E85 page to learn about that. E85 has mostly replaced water-methanol injection in terms of streetable octane-enhancment, but if you still want to learn about water-methanol injection, we've got a whole Water-Methanol Injection education page.
At some point you might exceed the power-handling capacity of the engine block itself. You can get into a whole world of engine rebuild options, including forged pistons and rods, upgraded bearings, cams, valves, and lots more. Many times cars with built blocks boast even larger turbos and correspondingly large fuel systems to deliver the fuel to go with high boost levels. We discuss some of the many options at our Extreme Modifications page.