Yes, you do have to declare it. Otherwise if you have a bump and have undeclared mods on your car then it will eventually lead to other people with the same car as you, us, having our premiums bumped because we will be then seen as higher risk. At the moment we have a good rub of it as these cars are still too expensive to buy, run, tune, insure and maintain for the idiot brigade.
Price change will vary depending on insurer, usually just an admin fee which shouldn't be more than about £25 though. An induction kit is going to make such a tiny difference to the power output of the car that any change in premium will be negligible. It depends how mod friendly your insurer is though really...