the filter is actually sized well for this engines needs, and unless you ride in extremely dusty conditions, a "normal" (a I have seen during replacement) interval, coincides with the valve adjust intervals well..
also, the filter you chose, is a good filter, and I personally would continue using that exact same one, it's cost effective, compared to OEM, and works.
I've used K&N in all my bikes, but I won't on this one, even tho I know how to "correctly" oil them all... the K&N for the zx/zg 1400's, comes pre-oiled... o never add oil when it' "new".. you will have issues, and a messy air box...