There is a point where more compression needs a very complex dome shape with big valve reliefs which masks the valves during overlap. Torque goes up, but peak power doesn't because high RPM VE is down.
This is worst with a "real" hemi like the Gen-2 426, and somewhat in BBC, Ford 385, Cleveland because of the valve angles. A wedge with nearly vertical valves and long stroke can run CR than these.
It also varies with bore/stroke ratio: longer strokes have more compact chambers for the same displacement and static CR.
You won't get the same benefit with lower CR, but I assure you that sidevalve engines with 6:1 CR and huge cams like methanol a lot.