any non OE change should strictly be declared to the insurers so you are fully covered.
you
could argue that its a small, hidden (within airbox) part that gives little or no gain so its low risk and prob not an issue if you dont declare.
However if you have other modifications (declared or otherwise) I would expect an insurer to go over you car checking.
We all assess risk in dfiferent ways though, so take the facts, know the rules and make your own judgements.
the restriction for the OE intake isnt just the filter,so the gains for a filter only swap just arent there.
by all means try it but dont expect anything near 20 bhp