If you can take your intercooler out, flush it, and reinstall it then you'll get a nice performance boost (and no need to declare it as you've merely given the car a good service). I used the following process:
Remove intercooler. Gently hose dried mud from lower 1/4 of it (do not use a pressure washer as it will wreck the cooling fins).
Lay intercooler on its back and fill with paraffin. Leave for a few hours, with the odd gentle shake to slosh it about
Empty into a suitable container. It will come out black.
Repeat with remaining paraffin
Drain, flush with water until it runs clear
Leave to dry thoroughly (IMPORTANT - if there is any water or paraffin left it will damage your engine)
Refit