Ok, let me set one thing straight right here and right now. Putting a K & N filter on your bike will not add another 20 horsepower. Heck it might not even add a single horsepower or pound of torque. Ok, that said, why would you want to pay a premium price for an air filter?
K&N KA-1596 For 1500 Nomad & Classic FI
How about pay once and never again? The stock Kawasaki air filter costs upwards of $40 dollars a pop from your dealer and only lasts 12 thousand miles. The K&N is a "clean and oil" type of filter guaranteed for one million miles.
The K&N probably "does" flow a little more air than the stock filter (or at least it has the capacity) but if you've ever checked out your filter housing you know air is coming into it through a fairly small tube (which is inhaling heated air from between the cylinders), runs through the filter then through another plastic tube (a warm one) that runs between the jugs and over to the right side of the bike...then the air makes a turn and finally gets into your carb or throttle body. Whew! So much for flowing more air through the filter.
The K&N part number for the Vulcan Nomad & Classic 1500 FI is KA-1596.
For the Vulcan Nomad & Classic 1600 the part number is KA-1603
You can order the filter through many auto supply stores or online.
If you "really" want extra performance, check out the Thunder Products or Barons Custom websites for a filter relocation kits and Techlusion TFI, Cobra FI 2000 and Thunder Power Commander for add on computers (for FI bikes). If you want to add a bunch of power on the cheap, check out the "Coyote Kit" or "CaddmannQ Mod" elsewhere on the Fixit site.
Bottom line, a K&N filter will set you back a little more than a stock one when it's time to change your filter the first time. After that it's a little water, a little oil and back into the bike. Cost? $0.00 and you've gotta love that!