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In general intercooler upgrades make very little to no difference for most people. They’re popular but generally the last thing you might want to upgrade.
So I’ll start with the intercooler placebo effect. One of the top reasons people believe an intercooler upgrade made a difference for them is because the new intercooler is clean and the old one is full of oil. The oil in an intercooler is a natural consequence of turbo seal leakage and pcv/ccv cycling oil back into the intake. The oil lowers engine out put by insulating the intercooler and lowering its heat exchang ability and as an octane reducer. Oil in the combustion chamber lowers effective fuel octane causing the engine to pull timing, to prevent knock. Extra heat also reduces the knock threshold..
The way you know if an intercooler upgrade will benefit you.... log your car, and clean your stock ic.
So what you’ll see in logs if you need an ic upgrade is if you do full throttle pulls on the highway, you should see your intake air temperature plateau. If on successive pulls you see iats continuing to rise and you see the engine pulling timing and mass air flow falling on,y then would an intercooler make a difference.
I’m going to state in my experience with many upgraded cars with big turbos, that most of the time the stock intercooler is fine until you run a big turbo too far out of its efficiency range and it’s blowing progressively more heat.
Bottom line Unless you log the car you won’t know if its money well spent upgrading the ic.
Frankly going catless will have a bigger impact than an ic upgrade. Lower back pressure and reduced exhaust temps means you can run the turbo harder and produce more power. Unless you have weak valve springs which doesn’t seem to be a general issue on bmws,
Oh and one other thing to consider... typically the bigger the intercooler, the more volume there is to pressurize in the intake... which means increased lag, yes adding a bigger intercooler can increase lag.
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