Just had unlit B road run with one nightbreaker and one normal bulb on dip - nightbreaker side is whiter that the standard bulb and certainly brighter (not staggeringly so but worth the £20). Suspect most benefit will be on main beam.
This fits in with the view that lighting is not staggeringly brighter but is more dazzling to other road users. I would alos be very concerned about the fact that I am invalidating my insurance.
I think they'd be hard pushed to claim you invalidated you insurance by fitting a standard wattage bulb! No different to people who fitted halogen bulbs in place of plain old filament bulbs in the 70s.