Well I've never known a performance air filter (such as K&N) which is a direct replacement for the original filter placed in the original induction system ever been insurance loaded. All air filter have different flow rates.
Your could forget to fit the air filter at the next service. Does this count as a mod?
To be honest I've dealt with a number of insurance companies over the years and have declared ALL mods (see below) and have never been refused or loaded.
Honesty is the best policy. Declare all and fear nothing.
The bottom line as far as the insurance companies are concerned is if their risk is adversely affected. e.g. major power gains, theft desireability, repair costs.
Declared Mods for Strada Abarth:
strut brace
ptfe steel braided hoses
uprated brake pads (did have TAX.OX disks as well but dumped them as a waste of money)
K&N panel filter
motor sport safety decals (ignition off, yellow 'cut this battery wire' tape, etc.)
brake air cooling dams
driver race harness
race proven alloys (Revolution RFX - 4 spokes)
coolent catch tank
fuel pump/oil pressure overide
my design throttle linkage with dual throttle return springs (sounds grander than it is
Declared Mods for Uno SX:
lowered & stiffened suspension (SACHS)
ptfe steel braided hoses
uprated brake pads (did have TAX.OX disks as well but dumped them as a waste of money)
K&N filter
free flow exhaust system
additional throttle return spring
wider 185 tyres on Fiat after market rims
driver race harness
motor sport safety decals (ignition off, yellow 'cut this battery wire' tape, etc.)
oil cooler
oil temperature guage
When both cars were insured with Prudential and because we were heavily into track days and sprinting I also PRE DECLARED the following mods that I might apply and got their approval in advance with no policy loading or further notification required
roll cage
passenger harness
more safety decals / blank white squares for competition number mounting
re-routing of fuel lines
and loads of other stuff.
what WAS NOT allowed was:
cams, pistons, turbos, nitrous,etc.
fancy paint jobs declaring I'm a car with performance worth nicking, etc.
fancy bodywork, spoilers, wings etc.
All in all I personally found that if you are honest and don't push the boat out too far then many insurance companies will respect your modest and purposeful changes in line with your personal vehicle usage and requirements.
Putting brigh blue induction and coolant hoses under the bonnet for display at car shows etc. is not self evident at normal roadside and does not significantly put the insurance company risk factor up. If you genuinely do motor sport (off road of course) in your car and you want better brakes, cooling, SAFETY etc. then most decent companies will respect this and have no problems.
Age, offical club membership and general declared intent play a major part in insurance quotes/acceptability. They DON'T WANT BOY/GIRL RACERS.
Declare all. Ask to speak to the underwriters (you may not be able to do this directly but you can asked the desk agent to contact the underwriters).