Topdog I know this isn't the same exact topic, but go give this a read for my take on the matter.
Charge pipe sizing & differences: - e30tech.com Forums
On top of the boost onset difference being minuscule, it is also not related to power output. You'll make more power with no boost if your charge pipes are high flowing over 1 or 2 psi of boost on restrictive piping. Building boost at the end of the day isn't the goal. The goal is flow. You could build
tons of boost
real fast if you went with tiny intake valves. You'd get the best boost onset in the world. Boost would come on like never before. You wouldn't make any power though. There's a difference between getting the turbo up to speed to flow, and "building boost" just for the sake of getting pressure.
Now I know you asked about intercoolers and I'm talking about charge pipes in that BMW thread, but a larger intercooler is even more important, and even less of a "problem". The larger intercoolers tend to flow a lot more and have much less of a "pressure drop" across them. They cool better, and they flow better, and they only add a tiny bit to the actual volume of the intake track. And if you use my formula in that linked thread you can find out in seconds how much of a difference in time it will take to make the same amount of boost if you change the intercooler volume (if that is something important for you to know).