Throttle response is likely the biggest gain as Typos1 has already said, and if done correctly as also said.
all stock equipment is always a compromise, it is never made with JUST performance in mind, so power gains are definitely possible, as is improved economy, and better torque, but much of the gain potential is computed out and countered by the MAF.
if we were to lose the MAF, run on a stand alone ECU, then the right air intake can begin to make significant gains, but not very practical to the wallet, and hardly necessary to achieve the outputs we can have before ripping the gearbox in half!
and my final point really, particuarly with the JTD's, is that the stock airbox and filter element if clean, does not prevent the car from being mapped to a sensible cap limit anyway, they will flow enough air to exceed 300ftlb no problem
however, what i wish to achieve with mine, is something that in as direct and short a route as possible, get as much Cold air in as i can, the MAF will still do its thing with outright air quantity, but if i can significantly reduce the intake temps by better materials, routing and insulating, there should be the scope for a broader spread of torque and with improved response and efficiency.
Intercooler mods are similarly uneccessary, but i'm for anything that will get the intake air cooler and denser.
and they'll look nice!
i'm not so bothered about the noise, it will drowned out by my diesel clank anyway!!! ha ha!!!