Looking inside the intercooler of my '06 MINI Cooper S when I had it off recently, to replace the supercharger, I noticed that the inside was coated in a thin film of oil. This decreases the efficiency of the intercooler by fouling up the heat transfer between the intercooler fins and the air flowing through it.This is the solution I came up with.
Disclaimer: I don't know how your intercooling system works. This How To won't do damage to a first generation MINI Cooper if followed properly but there could be a slight difference in your system that could cause problems. It's unlikely but possible so read the Instructable and then make a decision based on your situation. It is also possible that the inside of your intercooler may never need cleaned. In Gen 1 MINI's the crankcase vents into the air system, which means oil vapor and mist gets into the air system, making a mess of things.