Has anyone taken a look what engines CX operates? The 340-300 as the CFM56, there is no other choice. The 777-200/-300 have the TRENT 800 (RR), the 330-300 has the TRENT 700 and I expect the 747-400 to have the RB211. This leaves CX with a nearly complete RR-powered fleet. From this point of view the 340-500/-600 makes sense as they are TRENT 500 powered. Adding the GE90, which isn´t performing very well at the moment, would mean producing somewhat more costs due you have to store parts for this engine. The TRENT 500 has some essential parts taken from the other TRENTs which would mean reducing theses cost.
Just my 0.02 cents.