I think it is more of an incremental gains discussion. I have seen many TTs where the spread on the podium is a couple of seconds. The 1 km/hr for increased efficiency due to less chain friction caused by bending a chain around a gear or misalignment seems high.
Having said that, I use a 54t and most of the time trial racers use larger chain rings. There have been courses with terrain features that make the larger chain ring useful in addition to keeping the chain off the smaller rear cogs. For a particular race, the goal would be size the front ring and the rear cassette to match up with a rear cog to reduce friction that works for most of the TT at the desired cadence.
At the track, pursuit riders and riders going for records use larger rear cog and much larger chain rings to reduce drive train losses.