The thread has stopped gaining traction and it shows that air filter is not given as much importance as engine oil.
My experience with air filters is limited to pick/power, filter life (at which it chokes up) and maintenance (cleaning required frequently or not), not about the filtration efficiency. I've found paper-based filters (very hard to find nowadays) to provide very good RPM ramp-up and longer life than cotton (namda).
Used Nitco (Malaysia) A-110 (will fit Mehran and FX), cleaned at 10,000 km and then at 20,000 km and at 25,000 km it choked up and then no amount of cleaning would do any good. By choking up I mean when power demand was placed on engine it tried to stall but was fine on idle. What I really liked that performance (of engine) was consistent throughout the 25,000 km. Until the day it choked, there was no need of cleaning it (didn't try washing it once it choked). Cleaned at 10,000 km intervals only because I get the car washed every 10,000 km and it is standard practice of service centre staff, not because there was some deficiency with the running of engine. Since Suzuki F8B has a standard air cleaner replacement interval of 40,000 km, choking of filter at 25,000 shows how dusty our environment is. I don't know anything about the filtration efficiency of Nitco A-110, though. (Although there was no dust build-up inside carburettor).
After that, paper-based filters ceased to be available so switched to Guard cotton-based. Local mechanics tell me that CNG doesn't run smooth on paper-element air filters and that's why cotton filters got popular. I ran all those 25,000 km primarily on CNG. Petrol was used only occasionally. Any ideas?