On our diesels "Performance" air filters and intake mods including intercoolers that promise greater airflow will generally have the a detrimental effect on performance.
If you look at the airflow as measured by the MAF as standard and then compare it with the modification you'll find that often the airflow actually drops instead of increasing
As the airflow as read by the MAF controls the fueling, this means less fuel is injected.
Recently looked at a car that was experiencing poor performance, lo and behold it had a "performance" intercooler fitted. I've come across this a few times now.
Alter anything to do with the inlet system on the diesel that affects the airflow and you're most likely to be wasting your time or giving yourself less performance not more
Russ