Stallie,
Don't forget that the B747 series engines can equate to big turboprops and they hate high altitudes. The efficiency of the blades dies in the bum but is partially offset by the tip shrouding fortunately provided by the nose cowl.
Turbojets on the other hand, love high altitudes.
As has also been posted, the tropopause is all over the place depending on the location of the jet streams among many other factors. Pull the sigwx charts from Aust BoM and look at them over a few weeks.