You forget that the i30N already has an intercooler that works.
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You can see that the stock intercooler does a good job and the intake air temp difference is about 5°C. This gap will obv increase when running a tune, etc.
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You are basically right, I just wanted to mention that categorically stating that better intercoller doesn't count, and it's negligible, and it wont give you extra hp that is not precise.
The original IC is fine until you don't drive like a mad, you are a gentleman on the roads.
This is not the most typical behaviour with such a car. People tune these kind of cars. People wanna make it faster, more powerful. People lose licenses with hot hatches, unfortunately. It's hard to stay below the speed limits with hot hatches and sport cars, and people are a little wicked including me. I'm not happy to say that.
However the original IC becomes the weak link in the system, when you start to abuse the car, you go track, or you tune it. In that case you have to replace it.
Otherwise it's your choice, I'm not sure if it's worth the money. That's why I said "hmmm".
..but it will not add a significant amount of horsepower...
I said the same, 275hp to 285hp isn't very significant.
And it's quite expensive for the advantages. But it's something.
Like intake air filters. Also not so significant tuning only few percents. Still people keep replacing it, because it's something.
Although the IC is quite harmless tuning, while the oiled-cloth "perfomance" air filters have significantly worse filtering performance than the original one. Your intake air will have bigger dust particles and that's not good.