Some companies as long as they aren't performance enhancing mods... Exterior mods, stereo's, aftermarkets speakers and such like don't actually have to be declared.
For example, say you got a stainless steel replacement exhaust for your car and is different to that, that came with the car as standard, wouldn't need declaring.
Yes, suspension mods etc... Should be declared and it's stupid that they make the insurance more when they are actually making your cars handling better, should bring it down. I know what can and doesn't have to be declared.