It depends heavily on how much your house showers, what the water temperature is at the inlet to the house, and how ideal your plumbing is. The efficiency is highest with a vertical unit at least five feet in length, and that’s really just down to your house’s layout.

Note the manufacturer of the GP's device claims a 33% energy savings. You can get a hybrid heat pump hot water heater with a coefficient of power of 4, for a 75% savings. If your water heater is in a garage, it is likely a simple retrofit. (If it is in a small closet, it will require a duct).

Ignoring installation, the heat pump water heater costs more than those devices. With installation, I imagine it is cheaper.

Given the coefficient of performance of heat pumps is a function of the difference in temperature between the two mediums, if you can use heat recovery on the shower water you can boost your COP further by having both. I'm in the midst of this exact system being installed: it'll be extracting hot, humid air from the bathroom and receiving pre-heated mains water via a reclamation unit.

The main thing with EAHPs is finding a plumber who is cognitively flexible enough to fit one. I'm on number six and it is finally going in next week.