There's something I wanted to say about the dusk speck in a Knuth notation number of eyes versus torture for one person; something as light as a speck of dust wouldn't even register, it's noise level, in practice doesn't affect someone one way or the other. A bit in the same idea that you need a signal to reach a certain strength to make a neuron fire. So to make it work, you'd need something that at least makes a difference, even the smallest of differences, in terms of pain.

Now with that being said, different people have different sensibilities. This may even be more the case in the future indeed. But in the end, after writing this, can you still argue that it is preferable to torture someone for 50 years, rather than have an unimaginably high number of people people bearing some minimal pain ?