Probably right around 220 whp, 300 wtq (give or take).
Before you upgrade your headers, you might want to think about getting a larger top mount intercooler, fuel pump, and a 3 port electronic boost control solenoid along with a protune.
But if you do get tubular headers, be sure to keep an eye on the clearances. I have a set of GT-Spec unequal length headers on my 07 WRX and the front tube was close enough to my frame that it rubbed a little, so I had to get my dremel tool out and remove a little bit of material from the frame and touch it up with paint to keep it from rusting, but it's all good now. I also had an issue with the passenger's side lower tube being too close to my plastic under tray - some of the radiant heat from it melted some of the plastic, so I had to remove a good bit of that with my dremel tool too. In conclusion, a dremel tool will possibly solve all of your clearance problems.
Also keep in mind that unequal length headers maintain Subaru's signature boxer rumble and has slightly lower horsepower and slightly higher torque which is what you feel when you drive. Equal length headers will get rid of the Subaru rumble and make your car sound more like an average 4 cylinder car, but you'll have slightly higher horsepower gains and slightly lower torque gains. Listen to videos and whatnot and decide what you like. I don't have a massively huge turbo and I'm not concerned about big numbers or top-end highway pulls, so I'd rather feel more torque and maximize my daily driving fun... just some food for thought.
Welcome to the forum and keep us updated!