wow, this is really useful and helpful! Thank you for explaining some of this to me. This helps a lot.Let's back up and explain what it is an intercooler does before all this talking about heat soak. Turbos heat air as they compress it. Engines like cool air for peak performance. The intercooler goes between the turbo and the throttle and acts like a radiator, cooling the hot air coming out of the turbo by radiating heat into the outside air.
Heat soak is when the intercooler's no longer able to effectively cool down the air coming from the turbo, because the intercooler is too hot to effectively radiate the heat from the charge air to the outside air.
Cold weather helps delay heat soak because the air outside the intercooler is that much cooler than the air inside the intercooler, so it's more effective at pulling the heat out through the intercooler.
As your intake air temps rise due to the IC heat soaking, you won't necessarily notice right away that the car is performing worse, because it's using the knock sensor and pulling timing to deal with the higher intake air temps as seamlessly as possible.
Anybody saying their stock IC is fine with a tune, I'm not inclined to believe that unless they've checked their intake air temperature sensor readings and can confirm the stock IC is not heat soaking. Otherwise they're relying on their butt dyno and the ECU can too easily fool most butt dynos into not noticing heat soak as it's happening.
Think about it this way: the Si has a bigger turbo than the base 1.5T Civics, but the same intercooler. Running an aftermarket tune with higher boost means more heat, all thru that same intercooler used on plain regular 1.5T Civics that make way less power than a tuned Si.
Upgrading the intercooler won't make big sexy power gains on the dyno, but it will gain some power, and the big thing it does is make sure your car performs consistently with the higher boost levels from a tune.
Intercooler is going to be my first bolt-on upgrade to go with a tune. The rest of y'all can do whatever you want but IMO this is the smartest thing to do to ensure consistent performance.