In my opinion it's not worth not declaring them.
I certainly feel that if you've done some modifications which make your car faster, but you've added better brakes for example the insurers should see that as a positive, and rather than penalising you twice for 2 modifications, they see the better brakes as a positive and use it to offset the premium of the performance upgrade.
I think it also depends on your age and the type of car.
If you're driving a GTR for example, it's already a performance car, so the mods will add little in terms of desirability as it's already a desirable car. If you're driving a saxo where you add more than double the car's value worth of mods, then I accept that the premium should undoubtedly be higher. It's not always fair, but that's the way I think it is seen, but remember it has to be economically viable for the insurers to insure you - if it's not why would they insure you? It is a business after all.
I've always declared every tweak I've made, bar a geo set up. New brakes, stereo, styling, braided hoses etc... all that yes. A lot of insurers I've been with didn't even add more than 10% - some didn't even see some of them as worthy of increase of premium.
Livingstones Warman were very good to me, as was Chris Knotts (but you have to be over 30 I think) and Prestigekeepmoving.
It helps if you're a member of a car owner's club as often they do discounts and deals.
Ultimately you still have to hunt around, and compare websites are useless.