I have seen geese myself meany times and assumed that there was an efficiency gain.
Turns out that 12% fuel saving has been achieved in tests with A380s. I assume that is for the following aircraft.
https://en.wikipedia.org/wiki/Formation_flying
"After
A380s tests showing 12% savings, it launched its 'fello'fly' project in November 2019 for test flights in 2020 with two
A350s, before
transatlantic flight trials with airlines in 2021."
"Certification for shorter
separation is enabled by
ADS-B in oceanic airspace, and the only modification required would be
flight control systems software. ... Commercial operations could begin in 2025"
Decent looking article here -
https://aviationweek.com/dubai-air-s...lying-together