I would get the "L" 100-400 lens, the quality of the L lenses is worth it.
It's not much different form the Sigma 80-400 except
push-pull zoom, not twist
zoom lock at any length
faster AF
whote not black , so less heat absorption
less reflection on night car racing (so internal reflecitons better controlled)
less star pattern flare from diaphragm on night car racing

I think for me it was worth the extra money.

But for the trip of a lifetiime?
I'd go with Canon "L" prime lenses if I could possibly afford it.