The saving would still be substantial as the heat pump provides a 4.14 COP, meaning 1 Kw of energy provides 4.14 Kw of hot water energy.
A gas heater at it's best is only .90 COP, even a on-demand heater has 10% of its energy is lost to the chimney. Hot water on-demand don't have a stand by loss so are better than NG tanks but that is usually less than 10% if using hot water regularly as tanks are very well insulated.