- "why is the limit imposed by pion pair production as opposed to electron pair production? the energies required for electron pair production is /significantly/ less than pion pair production."
I recently came across the figure below, which depicts the energy loss of a proton times 1/E (the relative energy loss per year), due to electron-positron pair production, and - at higher energies - pion production:
[Fig 1 (a) from V.Berezinsky, A.Z.Gazizov, S.I.Grigorieva "On astrophysical solution to ultra high energy cosmic rays", Phys.Rev. D74 (2006) 043005, arXiv:hep-ph/0204357]
As one sees, the electron-positron pair production leads to a loss and is the dominant contribution at energies below ≈ 1019 eV, but at higher energies pion production takes over, increasing the energy loss by a factor of ~ 100 which is the effect responsible for the cut-off in the spectrum.
Essentially the same is depicted differently in the figure below from Roberto Aloisio's talk, slide 2. I didn't hear the talk but a good guess is that the y axis shows the attenuation length of the protons in the CMB background. Again one sees the electron-positron pair production having an effect already at smaller energies, but it does not result in a sharp cut-off as the pion production: If energy loss would be caused by electron-positron production only, the proton could travel as far as a third the size of the observable universe.
For an excellent introduction into the physics of ultra-high-energetic air showers, I recommend Angela Olinto's recent PI colloquium, PIRSA: 08010000.