It might be worth looking at another car that has a twin intercooler to see how they are set up. I have a feeling that's the difference between the Audi S3's or Seat Leon Cupra R's, they went from a single to a twin intercooler and that's where they jumped from 210 - 225bhp I believe. I'm not sure about diesels, but on a petrol turbo, a rough rule of thumb is that if you can lower the intake temps but 10 degrees, you will get roughly a 4% increase in power.