Is that loss of power in the higher gears possibly due to the fact that the compression ratio, boost levels and design make it so that the intake tempts are getting a little too high? If the air gets hot enough it will be obvious when doing pulls all the way through the gears. Top Mount IC's aren't the most efficient as they sit above a big heater, the scoop as cool as they look only allow so much air through them and you can't get that large of a cooler under the RS hood. That is if you compare a top mount to a front mount.
I don't work with Subaru's anymore (just don't get a chance) but you might want to look at something larger then. Then T28 is about right though if you want a more street friendly less peaky car.