Let's take two frames, taken on the same film stock. One is 2-perf, the other is S35 anamorphic. For the sake of argument, the aspect ratios are the same, and the S35 frame is double the surface area of the 2-perf frame. You expose each frame to the same scene.
Of course, if you are enlarging each frame to fit the same screen size, the anamorphic frame will show less graininess, especially in the shadows. Per unit of area, graininess is the same, but an increase in surface area buys you more latitude, practically speaking.
So, by how much would you have to overexpose the 2-perf frame to show the same subjective graininess as the 4-perf frame? Is there some kind of rule-of-thumb that you can apply between different frame sizes?