The bigger nerdy science behind the squish gap also has to do with thermal efficiency, swirl velocity, and combustion efficiency.
.8 to .9mm is considered "ideal" because it strikes a balance between how tight the gap is and potential clearance issues at high rpm. Ideal squish gap in theory is zero, but due to physics, that isn't possible. For many "race" builds people will run gaps of .5 to .6mm.
For swirl efficiency and velocity it goes back to one of Bernoulli's principles. That tight gap forces the entire volume of charge in a smaller volume of cylinder. This causes the charge swirl velocity to greatly increase. Faster swirl = better fuel atomization and mixing and faster flame front propagation = better bang for your buck.
Combustion efficiency is related to the swirl and the gap. Since the tighter gap forces more charge to the center, there is less charge swirling on the outer edges. The further the charge swirl is away from the center the slower it spins. The slower it spins the less energy it has, and that lack of energy creates a temperature differential in the charge that causes it to burn more slowly. Often resulting in as much as a 25% loss of fuel charge going unburned. The tighter the gap, the lower the loss of efficiency.
As for the torque increase, you feel it more at lower rpm because the increase in cylinder pressure means more torque. As rpm increases the lower the effect is felt because the combustion process is happening so much faster.
