Your logic is flawed with this tuna hp. The G600 is still more efficient than the G550, with the G600 having 2500 pounds less fuel capacity. At M.87 (HSC) the G550 has an advertised range of 6000 nm. At M.85 (LRC) the G600 has an advertised range of 6200 nm. If you bump the speed of the G600 to a matching M.87 the G600 will fly the same 6200 nm., on 2500 pounds less fuel. Again...the G600 is doing so pushing more mass through the air, on less fuel, going the same distance, at the same speed.
The G600 fits the desired customer niche. I buyers want more range, that's when they'll step up to the G650/ER.
Of course its more efficient than the G550, but not by nearly enough given the 20 year difference in entry date.
At least as far as cabin size and speed, G450:G500 as G550:G600. The G450 and G550 has the same cabin cross sections, and they represent a certain length without a crew rest and with a crew rest. The G500 and G600 have the same larger cross section and are slightly longer representations of not having and having a crew rest. G450 and G550 LRC is Mach 0.80. G500 and G600 LRC is
EDIT: Mach 0.85. So what I'm saying is to some extent, comparing the G450 and G500 should be analogous to comparing the G550 and G600.
Well the G500 gets 17.5% more nm to the lb than the G450. The G600 supposedly gets 2% FEWER nm to the lb than the G550.
I mean I could be wrong but that would make the G600 the worst plane Gulfstream has built.