I have seen them but not worked on them.

I can't see them being significantly more energy efficient than similarly equipped, pump power wise, tubs. The biggest heat loss is at the surface. A proper fitting cover, high quality is key to locking in heat.

I do like the concept of the interchangeable backs but that would remove calf jets, something some people want or need. The packs help to reduce the number of holes in the tub, which reduces the number of gaskets to dry out. It also reduces the plumbing lines in the cabinet but outside the shell. Most tubs have lots of spray foam which serves to insulate the sides some but most importantly, holds the plumbing steady in shipping so vibrations don't knock anything loose.

Figuring out energy usage is tough. The things to consider are pump configuration, i.e. how many and how big, economy vs standard modes of operation, accessories, your usage patterns, etc...

My customers have told me in the past that the bill usually goes up between $25 to $50 per month.

Scott