A clean filter when brand new is usually on the limit of their filtration specifications, this is because as they are used and they get dirty their filtration improves not decreases, yes this means that the airflow drops but this is accounted for when the filter is specced out, under normal driving circumstances a paper filter won't need replacing due to being clogged with dirt unless you have gone past the filter service interval.
NB: If anybody wonders how stainless steel cone filters factor into this their not in the same league as paper/cotton/foam. Unless you plan on driving into a chip shop their just bling.
It is not about the filtration, it is about the consistency of airflow, yes a paper filter offers the same filtration and as the pores get blocked it does filter better but reduces airflow and therefore performance.
In carbs it means fuel enrichment and even worse performance, but on fuel injection or turbo charged engines it's just loss of power. The more the engine is dependant on good airflow, the more power loss you will get from a stock dirty airfilter.
Stainless cone filters as you say are generally rubbish, as they sit in the engine bay and draw in warmer air and get dirty faster as well as having less surface area to filter the air. At least in a stock airbox, the filter gets ducted air, usually from an inner wing, that is cooler and cleaner. An oiled panel filter and exhaust (+ remap) usually see's a reasonable gain on turbo charged cars, allowing better breathing in and out. They make enough difference to be considered an essential part of a stage one package on most modded cars.
Just fitting a panel filter on a normally aspirated car with no remap generally makes little difference, maybe 1-2 bhp at the top end, but the main advantage is the washable aspect and consistency of airflow. If you aren't modding the car for performance, and don't drive it hard, you will see no real benefit in fitting one.
Because they use the stock airbox there is no improvement in noise, and no real improvement in throttle response on a non-turbo car.