I just thought of something that might help...
Back in my crazy horsepower days, I drove a Mitsubishi Evo-8, Turbo-Intercooled (The Subaru STI may have the same contraption I'm describing...)
It had a reservoir, pump, lines, and spray nozzles activated by a rocker switch on the console, to spray water on the intercooler. The idea was to get a volume of cooler air so the ECU would allow more power through the A/F ratio when at full-throttle.
Instead of using water, someone on a related forum had the idea that Isopropyl Alcohol is readily available from pharmacies in quantity, did not damage the lines, pump, or intercooler, and provided much better evaporative cooling. I would buy the highest % Isopropanol in for about 99-cents per quart and kept that in the reservoir tank.
The cooling effect was significant and resulted in the goal of better A/F ratios with more power at full boost. It also did not freeze in the Winter, so it cleaned the fins if they became clogged with road grime or snow. Since the pump whole setup was essentially a big windshield-washer spray system, it would tolerate the washer fluid and ultimately the alcohol.
You may give it a "shot" to see if it helps.
Use caution as it may be flammable if highly concentrated and/or used near open flame
RH77