K&N's XStream Series MX filters. On the dyno,  have seen noticeable difference horsepower which has for sure been an advantage on the track!  The added air-flow delivers the cheapest power ad-on available!

KA-2506XD Air Filter Specifications
Flanges: None
Height: 3.75 in (95 mm)
Base Outside Diameter: 9.5 in (241 mm)
Top Outside Diameter: 5.5 in (140 mm)
Weight: 2 lb (0.9 kg)

Product Box Length: 8.5 in (216 mm)
Product Box Width: 8.5 in (216 mm)
Product Box Height: 3.63 in (92 mm)

Manufacturer Part Number
Kawasaki 110130011
Kawasaki 110130722

K&N KA-2506XD fits the following:
2016 Kawasaki KX250F 249 - All
2015 Kawasaki KX450F 449 - All
2015 Kawasaki KX250F 249  - All
2014 KAWASAKI KX450F 449 - All  
2014 KAWASAKI KX250F 249 - All
2013 KAWASAKI KX450F 449 - All  
2013 KAWASAKI KX250F 249 - All  
2012 KAWASAKI KX450F 449 - All  
2012 KAWASAKI KX250F 249 - All  
2011 KAWASAKI KX450F 450 - All  
2011 KAWASAKI KX250F 250 - All  
2010 KAWASAKI KX450F 450 - All  
2010 KAWASAKI KX250F 250 - All  
2009 KAWASAKI KX450F 450 - All  
2009 KAWASAKI KX250F 250 - All  
2008 KAWASAKI KX450F 450 - All  
2008 KAWASAKI KX250F 250 - All  
2007 KAWASAKI KX450F 450 - All  
2007 KAWASAKI KX250F 250 - All  
2006 KAWASAKI KX450F 450 - All  
2006 KAWASAKI KX250F 250 - All