The +++BHP that K&N refer to is based on tests they did a very long time ago, on breathless american family cars with undersized paper filters…remember that they came out of hot-rodding in the US where doubling the power only required a better filter and exhaust.
Modern cars have paper filters that are properly matched to the required flow and so there is usually little improvement can be made without impacting on the engine’s life. However, taking the filter out of the airbox does still improve the sound…