The heat is instant... why we call them ''on-demand''.
The gallons per minute that they can deliver is based on the temperature rise needed and the wattage of the appliance.
While this link shows ''up to'' gpm; in general, for our area using well water would require the two bigger models for most homes... and if you undersize... you deal with it; while if you oversize... the unit just costs a bit more rather than take a lot more energy as the internals make up the difference.
https://www.amazon.com/Rheem-Tankles...1MR7Z39V&psc=1
At the bottom comparison, you would see the 18 and 24. The data on the package is much more specific as to flow rate based on the required rise in temperature. Both would likely need a service and panel upgrade.
I doubt when compared to a tank water heater that they would have a shorter lifespan or more energy consumption based on equal usage.
It is just not the way that I would go.
The cost of removing a functioning water heater, possibly moving it to a new location, and the upgrades would take quite some time to recoup when only the standby loss is taken into consideration. A simple time, or shutting of the breaker for longer periods, would cost less and create a large percentage drop in the standby power usage.