Rick,
Electric water heater efficiency is 3.413 BTU/Watt. That conversion factor does not change with age of the heater. If the power is going in the heat ends up in the water. It has nowhere else to go! With a gas water heater, there are flue losses, some of the heat goes into the water and some goes up the flue. As a gas water heater ages, soot and corrosion and lime build up on the heat exchange surfaces; then more heat goes up the flue & less into the water, so the efficiency drops.
As for warranties, my brother is a full time plumber. He commonly sells tanks with 8-10 year warranties. Look around.
Stretch