I'm not at all claiming that there's enough flex in an OHV head to account for 1.5 pts of compression.
That number is likely as old as replacement heads for flathead motors, which I could see flexing enough to account for 1.5 pts.
The thing is, most engines could get away with far more than +1.5 points without ever thinking about head material and so alum head makers can make that blanket suggestion without risking much. The +1.5 myth probably also helps sell alum heads because any performance gained would be from the compression, not the head material and no one wants to spend money for nothing..
However, head flex is 100% a thing. All heads 'lift' to an extent when under load, and all metal moves and deforms when under load. How much is highly variable, but alum tends to move twice as far as steel/iron for the same load, so in my mind the extra compression to account for deformation is the only thing which is based on science - but how much adjustment is needed? Likely not much if the engine is worth two ***** to begin with.