My advice would be to declare the mods to the best of your knowledge.
I was under the impression that for an insurance company to refuse a pay out due to an undeclared mod, they would need to prove that the part in question was a contributory factor to the incident. The exception to this is the case of the car being stolen, where they may insist that mods make a car more attractive to theives, so must be declared so they can risk assess accordingly. Perhaps this varies for company to company.
Things like exhaust and air filters shouldn't really make a huge difference as you're not massively changing the performance of the car. If they try to charge through the nose - let them know you won't be staying with them next year. You will probably hear: "Oh, I'll just speak to my supervisor and see if we can do it any cheaper..."