There are some odd things with your observations. Aluminium has a higher expansion rate than steel, 12 or 13 X 10-6 vs 6 or 7 in/in/degF. That's less than a tenth of a thou difference even at wild temperature swings.
I think the difference is mostly in the Pseudo V-Block design of the HS bedding block, not the material differences.
It could also be that ANY metal block (including steel pillars) will have that torque loss effect simply because the bedding is not compressed as the screws are torqued down. That's the main benefit of blocks and pillars. In other words it's the screws that are causing the loss aggregated by the V-Block design (which does compress a bit like a spring), not the clamped material.
But for the record, I make all my own pillars out of stainless steel too.
Good stuff. Lots to think about.