Short answer, no it would not make econ sense. On-demand hot water requires a larger gas supply line.

Unless your on demand unit is next to your kitchen, then you still have to run 40secs to charge the line.

Another alternative is a recirculating pump. It is located the furthest from the hot water tank and there is a return line that takes the hot water supply and pumps it back to the tank. Now you have hot water when you turn on the tap and no waste water.

It is usually set on a timer ( same time every day, like am, or pm), or an on-demand button. If it runs continuously ( like a hotel boiler setup) it will cost you $$ in heating.