I think it's best you're kept busy Alex, I don't like to see people loafing about.8mm id might be one for the future if you wave enought money at him. As it is you've got a sought after product with first to market advantage. (as a mainstream supplier)
The next thing you might look at is the heat loss from pole tails.Assuming what you are suggesting takes care of the hose on the reel the next major heat loss is the pole hose tail.(the taps get hot too and must give off heat).
So a hose tail that retains heat please.
Can you actually afford to run a hot water system if you're worried about the tiniest proportion of heat escaping from stupid places like a hosetail.
You should be making more dosh from working quicker anyway.
Worry about the main heat loss i.e. the tank (insulate it), the hoses (insulate them) and Gardiners will be a great help with the 10mm internal 'exotic' hose.
Women say: "worry about the pennies and the pounds will take care of themselves".
My motto is: " concentrate on the pounds and stop fluffing around with the pennies".