Efficiency of cubing with respect to DPM

I'm not sure what you're talking about:
% pot >< damage
% to your main stat will have the same effect for all classes, since the dmg formula
Multiplier * (4 * main stat * %potential) * (Attack / 100) = %pot * damagewithoutpotential
is the same for all classes. Unless you add to secondary stats of course, where you could say thieves and especially bandits are at disadvantage since they need alot of that to be able to equip stuff more than say bowmen, and warriors are at an advantage since they don't need dex at all.
But seems more you're talking about
% pot >< Range
Well all (classes/weapons) have different multipliers, their ranges will differ. This is (or should be) balanced by giving skills with more % to the classes with a lesser range multiplier.

Well suppose you have a hero with 4k str, a nl with 4k luk, db with 4k luk and say a mechanics with 4k dex.

Assuming they have reached the optimum weapon attack possible for their class,

then mechanics would deal a higher dmg generally than hero,nl and db, if we are comparing those 4k primary stats.