Aside from minor rounding errors for very low damage, a monster with x hit points and the insubstantial resistance type is exactly the same as a monster with 2x hit points.
So, why does the "insubstantial" quality exist at all? It seems to have no actual purpose. (If there's a weapon out there that does full damage to insubstantial monsters, it could be modeled as a weapon that does double damage to certain types of monsters.)