Welcome to our forum.
Lets see if I can work on a new approach here So, you put 15kw hours of electricity into each spa. Would you agree that the spa that loses the LEAST amount of heat (energy) through the cabinet (to the atmosphere) is the most efficient?
I'm not. You've done nothing to explain your position, or reply to my questions, but you'll walk away saying your happy to stil say I'm wrong?
Foams, fibrous materials or evacuated spaces (vacuum fer da unedumacated) are used to reduce convective heat transfer by stopping or retarding the movement of fluids (liquids or gases) around the insulated object.
That's how double pane glass insulates (except that it is usually N2 between the panes)