K & N filters are good for roadway use and I put them on my street bikes and cars because they allow more airflow with a small compromise in filtering ability. Off road however especially out here in Mohave County AZ, beside the rock and gravel hard pack there is a silt dust out here that is so fine it cakes without moisture like talcum powder and will get past a K&N filter compared to a well oiled, high quality sponge filter, and for that the oiled OEM filter workers better. Its so fine that you have to put filters on the end of all your drain lines or the dust gets right inside the carbs out here.
If I modify the air box I will swiss cheese all sides of the air box with 1 inch holes but will leave the top intact to deflect engine noise that increases allot when you remove the lid (pinging in the brain). The 1 inch holes (about 30 of them) will leave open up the air box plenty yet leave enough structure to the airbox to go back to OEM if the EPA laws become stricter (inspections in some state & Fed parks) or OEM conversion for resale.
So with your air box completely open what size main jet and pilot jet are you using, and how many turns out on your mixture screw and what position do you have the needle clip set at?

 

my air filter air boot combination is primarily done for sand riding . I'm doing all the little tricks i can to get the HP from a stock bore unit and still use pump gas.
if I'm trail riding, power is not the issue that it is in sand i oped for a UNI and an top open air box .
for some reason carb's and jetting seams to be a controversial subject , no one seams to be set up the same way and yet every ones got it right.
what i know is very few out run me ,mostly yfz's with mods and jacked up 450r's.
trail riding the ltZ out shines them all . my settings are in my sig,i do find a fare number of riders are using a 165-25 combination if fully piped, 155-25 ,if not fully piped ,needle on the thirded clip.

 

Oh somehow I missed your sig line but this is good info I can use as an extreme baseline and work somewhere withing that range, thanks. I made some needed adjustments to the front suspension. It turns out the tie rods length were different so the tow was off a bit and what was causing the shimmies. I decreased the preload on both fronts and increased the rebound by 1/2 turn to compensate. The front tire pressure was also high 8 PSI which I lowered to 3 PSI. Now the ride is much more plush and controllable without the lateral shaking and no more front wheel shimmy. I have always been a gear and clutch man but I do admit I miss the auto clutch on the OZARK and I realize now why the Rekluse auto clutch is so popular, never having to stall out even if you stop half way up a steep climb. The only way I could stall my Ozark was with the kill button. So I may consider a Rekluse system if they make one for the Z.