I've looked into this a lot over the years and there are two main points.
1)Older design cars often didnt have a good cold air feed to the airbox. In these cases putting a feed in, or installing a good closed IK with a feed did help. My old VX V6s in Vectra Bs benefitted a lot from a closed IK and cold feed. Modern cars don't have this issue as much. For instance Mercs have cold air feeds direct from the grille. Funnily enough, my wife's Yaris 2009 didn't have, the intake was directly in the engine bay, so I bought some good quality pipe and put a grille feed in. It made the acceleration a little smoother, but not necessarily faster. TBH on hers using super unleaded gives the most benefit.
2)There is NO flow difference between a NEW paper filter and a new K&N or Pipercross or whatever. There IS a difference when they are both dirty, the K&Ns flow more. This is made worse by the ludicrous oem air filter change intervals that manufacturers have to make their costs cheaper - 4 years or 40k miles. You are better off simply changing the OEM paper yearly or every 10k. For the sake of 15 or 20 quid.
You can get more induction noise with IKs or "performance" filters, but do you really want that on a diesel?
oem it is
