There are plenty of independent groups which have have tested and documented (Dynojet being one) K&N air filters against other air filters and K&N have always comes very close to the top if not the top for air filtration of fine dust particles and flow rate.
I have fitted them to all my bikes and cars over the past 25 years and in every instance have found improvements in all vehicles in which they were fitted. All the Fazers; 400,600 and 1000 I have owned have felt smoother with better sounding intake noise, but my present car, a Mazda 6 has shown the biggest difference in improved throttle response and slightly better fuel economy.
Even my wife commented on the improved throttle response.
The K&N filter in my 600 Fazer has covered over 80,000 miles and have been washed 3 times in that time thus saving me the cost of buying 3 OEM air filters.
The FZS600 runs rich from the factory to get it through the "Pass‐by noise testing of accelerating road vehicles", which is one of the reasons the engine was phased out as it could not meet the noise and emission testing regulations.