I have to say that usually the biggest effect of "performance" filters are just getting a better sound.
On stock car just changing the filter isn't usually doing anything really. Of course this can vary.
Usually better filters should be used on tuned engines, because the original filters and air boxes may constrict the air flow. Also it makes the sound bigger, so it is a way to tell public you have a powerful engine.
I have a Pipercross filter on my car in the place of the original air box / filter, also the intake pipe is bigger than original. But the main reason is still to make it sound better. Yes my car's engine is slightly tuned, but to be honest if I would put the original air box there, it probably wouldn't have a big effect. Also it still haves the original MAF.
About the torque. My car has better torque than original, but the torque is on slightly higher rpm's. This is Ford "Pinto" 2.0 OHC engine.
List of what is done and what also has the biggest effect on torque, works on most engines if built properly.
Lowered and ported head (bigger compression ratio, bigger ports allowing more air flow)
Pipercams "aftermarket" cam, a "mild" one, so gives something but not anything radical.
Modified MAF, for richer air fuel ratio. Not good for fuel efficiency, but whit original MAF this engine is just horrible to drive.
Also the exhaust is modified a "staight" 2", don't use too big exhaust on N/A engine, it kills the torque.
Of course the whole engine is rebuilt, putting a aftermarket cam to engine what have seen million kilometers isn't usually doing much.
Other very useful stuff what could have a noticeable effect.
Adjustable cam wheel, adjusting cam timing can have a big effect on what RPM the power is produced and how much.
Better exhaust manifold, for this type of engine 4-2-1 should do the job best.
Better, adjustable engine management system, for remapping the fuel and ignition maps. With them I could use the original inlet manifold with one throttle body, but even better with individual throttle bodys.
OR a pair of side draught twin choke carbs, ie. Dellorto DHLA 40's (motor porn). And a different adjustable ignition system.
And of course with better engine management system I could use more aggressive cam.
Or I could just do a turbo conversion, very easy with this engine, currently planning it.
Also when people say that aggressive cam kills the torque, it's bullshit.
Aggressive cam just moves it to higher rpm's...
And usually with more aggressive cam the torque is higher.