The turbo in the 09+ WRX is the same size as the turbo in the STi..
Headers would shift your power further to the right (less bottom end, more top end), which is not what you're wanting. A tune is about the best you can do (aside from spending thousands on a stroker). e85 would probably help out, and if you're worried about the clutch just don't tune for that much peak torque. You could also upgrade your boost control system..
If it were me I would get a good turboback exhaust, a 3-port EBCS, and a good MBC. Then take it to a tuner and tell them what you're looking for in the tune. If that's not enough then think about EWG or e85.. or both.