Clive, I think it is for a bare engine not attached to the airframe.
Yeah, but when you read the small print it ain't so obvious what you have:
GE engines "includes basic engine, basic engine accessories and optional equipment as listed in manufacturer's engine specifications including condition monitoring instrumentation sensors"
PW engines "includes all essential accessories but excludes starter, exhaust nozzles and power source for ignition system" [but the PW4164 weight includes the starter apparently]
RR Trent 768 gives Basic Engine and Dry Powerplant weights
That's why I think you need to try to find something that includes the all-in values.
Following up on Turbine D's remarks on total fuel burn and your earlier reference to the A330 performance data extracted from the AI Airfield Planning document, Boeing publish similar planning guides for all their aircraft (AI seem very coy about it for some reason - I did find a URL but you need to be registered in their Airbus World). Some of the Boeing planning includes P/L / range diagrams for several engines. Taking the zero P/L / Max.Tankage range is going to be as near as one can get to a "pure" comparison between engines as the fuel on board will be identical and the TOW will be OWE plus fuel so that powerplant weight effects are also included.
This gives:
B757-300:- PW 2040 4250 n.mls: RB211-535E4 4150 n.mls
B767 300ER:- CF6-80C2B7F1 7100 n.mls: PW4062 6950 n.mls
That is all the comparative data I have been able to locate so far