Just to nitpick a little: The peak efficiency seems to be around 2500nm, and it's pretty flat between 2000 and 3000. Above 2500 nm, you have to compare to distance/2, since you compare between 1 and 2 legs. Example: compare the efficiency at 3000nm and 2*1500nm. The cutoff is slightly higher than 2500nm. But not far from 3000nm. Coincidentally London-NYC is 3000nm. So anything longer than that could be split off (if you don't take a detour to do so).