Do you have an air filter gauge? It shows how much air restriction or 'drag' on the intake. A diesel engine is just a big air pump, pulling air in, compressing it, and blowing it out the exhaust. I'd think the engine/chassis designers knew what they were doing. If you design your own 'scoop' or other modification, be careful you're not pulling in too much water in rain or creating new issues. You can dramatically decrease any air flow restrictions by removing the filter and also the muffler. I mean what's a little dust in the turbo???
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI