I doubt they'd get much benefit from preheating the water in the
pipes. There's just not a lot there. However, it's not uncommon in
large buildings to install a "tempering tank" in the boiler room. It's
basically a large holding tank that allows water to reach boiler room
temperature BEFORE it hits the water heater. That way the water heater
only has to raise it from 70-80 degrees to 140.

I have a friend who uses old water heaters (the non-leaking ones) as
tempering tanks in all of his rental properties. He removes the
insulation and installs them in the boiler rooms. He figures whatever
heat they absorb is that much less heat he has to pay for.

I wouldn't recommend his method if they're putting it in an attic--at
least not an old tank.