What a load of bollocks.
The petrol engine runs most efficiently as with all engines at Lambda with petrol this is 14.5:1 AFR. Lambda varies from fuel to fuel so if you run methanol for instance you will run a lower AFR.
14.5:1 AFR is the petrol cruising Lambda but under load the engine needs more fuel, typically 13:1 AFR but on a turbo engine the AFR might be 11 - 11.6:1. So basically the preamble above is rubbish.
In my experience glossy advertising sells air filters and air filter kits. The cars I have experience with, Subarus, Vectras, Astra GTEs and Jags, none of them in standard or semi standard form benefit whatsoever from an "uprated" fuel filter.
I do run K+N panel filters simply because they are reuseable after cleaning but they do not make any more power than the manufacturers cardboard item.
If you look up the K+N technical pages you will find they list the filters that have actually led to a performance gain on a specific vehicle. If they don't list any gain chances are they are not making any unfound claims.
