The internals aren't that crappy. The 35R is a pretty laggy turbo with massive top end. Also, your 6-speed is not indestructible. I hate lag, but that's me. Also, you should know that on the freeway, the AWD advantage is gone. It's just added weight and drivetrain losses. You're also short 4 cylanders. There are going to be lots of cars that can bury you in acceleration at those speeds even if you're over 400whp. It's just the nature of the beast. If you want a high speed car or a drag car, the STi is a poor pick.
I'd first recommend picking up a nice clean miata, turboing it, and having a blast. They handle better than anything at 10x the price and a built motor with a turbo can push 250rwhp which when your weight is like 2300lbs, it's enough to shape a lot of v8s. For 15k, You could have one hell of a sunday driver. 2 cars also are nice in that when one is down, the other still runs.
Now, If you want to spend it on the STi, I'd go with a twin scroll solution. It's going to be expensive to get the plumbing you want but the result is less lag and a squarer torque curve.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD