Strictly any non standard equipment is a mod, be it a 3cm sticker to a supercharger but at lot of insurers won't count one or 2. Id just ask em if it would change premium.
Don't forget undeclared mods can invalidate your insurance.... so you can not legally drive it! Get caught and its bad news!
Its obvious to an untrained eye if using a blitz or px filter that are obviously not standard but some like cosworth look "normal-ish". You could allways keep the old paper filter, it only takes 2 minutes to change
I personally doubt insurance companies would check the air filter if you have a standard looking engine bay TBH, unless they suspected it was a factor. But they might try and use it as an excuse to not pay out as you had not declared it.
What would be interesting to know is If you follow the policy any non nissan part would count as a modification it would get silly, if your garage fitted a blueprint oil and air filter or non nissan aux belts it would have to be listed!