Push processing doesn't actually alter the film speed. The shift in the toe of the film curve is minimal to zero. All that push processing does is increase the mid-tone and highlight density, making an underexposed negative easier to print.(posted 8626 days ago)Take a good look at the published sensitometry curves for 400, 1600, and 3200 ISO rated films. NONE of them show any density much below -3log Lux-seconds. In fact, the curves for Fuji Neopan 400 and 1600 are *identical*.
The simple reason is that film naturally tops out at around 400 ISO, because it requires a minimum of 3 photons to expose one Agx grain.
Some recent research has shown it possible to 'dope' grains to acheive a single photon sensitivity, but there's no film on dealers shelves using that technology yet. Even so, this would only mean a true top speed of 1200 ISO.