I went back through the utility records for the only water heater replacement I've ever had (nat. gas). It was in a place I rented for a long time. It wasn't a necessary replacement, the landlord wanted to do it after the plastic drain valve failed. The tank still looked good inside and had little scale or sediment. The existing water heater was at least 8+ years old, don't remember the model. The new one was a Kenmore I believe (Sears install, wasn't my choice.) Gas use in the years before and after were unchanged averaging about 18 ccf/month during the summer. (Same old furnace with pilot the whole time, same showerhead, etc.)
There seems to be some confusion here about terminology: efficiency is not the same as recovery time or effective capacity. This is especially noticeable in electric water heaters as elements foul and burn out.