All you need is a boost controller and an EMS. Fortunately the EMS on the Focus is re-programmable unlike for a bunch of non american cars.
But changing the boost also requires you to change a few things. The cheapest being the head gasket and get a thicker one, which will reduce the compression ratio. Of course you could buy lower compression ratio pistons but that will run you about $1K.
Using higher octane fuel will reduce pinging and detonation. A bigger I/C would be a good idea and even better if there's a fan on it. Bigger injectors, more powerful fuel pump.
Personally I wouldn't increase the boost just for the heck of it without building the motor a bit. What I would change is when the boost comes on. I noticed that on the Focus, the boost doesn't always comes up but only when you seem to need the extra torque/power. My track car has a turbo kit (max 18 PSI) and the boost kicks in a soon as I accelerate, regardless of the gear I'm in. Then again, the setup is completely different and the turbo isn't stock.