Just to add to the gearing argument ?

It is possible to get a faster top speed in each gear, it isn't completely governed by the gearbox.
For example if you turbo charged an 8v gti you would probably put a few mph on the lower (shorter) gears, on the higher (longer) gears there would be a more noticeable difference, ie. top speed.
The only way to increase speed in gear is for either to change the gears or tune the engine to produce a lot more torque top end. As the gear is only a multiplier of the torque the engine produces.
Just to add also an air filter won't really do enough to increase top speed in gear ?

So in general, a huge increase in torque will increase top speed in each gear, more the higher the gear. As mentioned earlier, rather than being that noticeable the car will reach the top speed much quicker.