Well, 1st of all to determine your filter is dirty or not, just simply point the filter towards the sun or a light source. If can see through the lights means your filter is ok and no need to wash. Some times after you washed, the filteration is not as good as half dirty cos with "dirt traps dirt" concepts works on K&N. Unless it is wery dirty then you need to wash it and re-oil it. I've been using K&N 10 yrs ago and the performance of this filter is excellent. I just wash it with dish washer cleaner and re-oil it using ATF then put it under the sun for 1 hour to let the filter element to soak the oil evenly. It works fine for me.
Weather installing the new filter can reduce or increase FC is depends on your driving behavior. If your driving style is smooth, and consistance, i would say your car FC will reduce and overall performance will increase due to more air (oxygen) to promote better combustion and less pollution.
Just my 0.22 cents:D