You can make more power, but the case here is more for repeated power. As you may know, an ideal gas undergoing adiabatic compression will heat up significantly (as the turbo compresses the air). This reduces the density, so the intercooler, as the name implies, cools down this air in part to increase its density. That's probably why the Subies could make more power with an IC.
Now it is true that over time an IC can get hotter and hotter from repeated pulls, and that's primarily what we are considered with in the GTI. Eventually we suffer from heat soak, and the ECU cuts a ton of boost and power to reduce the temperature of the air going into the intake manifold. Even before heat soak, you can lose considerable power just because of the lesser density of the hot air.