I'm not sure if the whole 744 fleet has had the upgrades, but some of the Pacific 744s were upgraded from RB211-524Gs to -524GTs, indicating the installation of new Trent technology that lowers overall fuel burn, thus increasing MTOW/range. The later-build 744s delivered to QF definately had it (OJS, OJT and OJU) which were sent predominantly on Pacific runs in their early life-- some others may have been upgraded as well. This went some way to redressing payload restrictions (on 747s of both engine types) which were a thorn in the side of QF's Pacific services for some time until the ER came onto the market (QF 93/94 in particular).
Despite having good mature fuel burn, the RB211 is still less fuel efficient overall -- it's just that toward the end of very long sectors does the RB211 start to burn less fuel than its counterparts would on a comparable flight. 22-24 missions and the robust nature of the engine see it better suited to more fragmented time in flight and the greater number of cycles typical of European ops.
It is for this reason that when QF had the opportunity to take on some GE powered jets in the late 90s -- VH-OEB, C and D -- that they were classified as 'Pacific' 747s rather than part of the Kangaroo fleet, which involved a lot of rotations between Australia and Asia as precursors to the longer Asia-Europe sectors, where the RB211 is ideal. The GE lower overall fuel burn is better suited to longer non-stop sectors, SYD-LAX/SFO for example. JNB is also subject to some very strong prevailing winds at certain times of year; most of the time it is much of a muchness in terms of which engine is sent. The prevailing factor seems to be the premium seat market. I believe EZE is really only achievable non-stop with ERs under normal circumstances but happy to be corrected.
The arrival of the ER into the fleet (and subsequently the A380) has changed this somewhat; RRs rarely crossed the Pacific besides the 2-class aircraft on QF25/26 and what few services weren't covered by ERs were done so by the 'Ugly Sisters ' in OEB etc. Only recently has the 744 fleet been deployed almost haphazardly as 3 class services have been truncated, more QF11 and QF93 services are turned over to the 388 etc.
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