Wouldn't running Solid lifters (Roller or Flat Tappet) be basically like running a really Mild Solid cam? And if so, why would the lash be necessarily any different? I'm mean, Why do you lash the valves at only .006? What about aluminum blocks and heads?

Solid or Roller, the metallurgy of a solid cam is as hard or harder and the Hydraulic ramps have a slower Velocity, usually solids have the advantage in that they have the capability of remining stable with faster valve action /ramp rates