You can sometimes bundle mods together.
My current ones have already said they would count a spoiler and body kit as a single mod. And any changes to brakes as modified brakes. Just brake lines or Massive calipers and whole new setup would be not be seen as different.
You may find you can be honest but still do yourself favours (e.g. declare N1 aero dynamic kit as 1 mod as opposed to bumper,skirt,spats,spoiler). But if in doubt ask them!
Your HU may already be covered. I've had previous policies where the the blerb just covered x amount of in car entertainment.
Also check the mods are actually covered... admirals standard policy for example does not cover the repair of the modification.