Wow great insight, thanks for posting the figures, they are so hard to come by freely on the internet.
Like I said, I don't believe that comparing the two aircraft at the same speed is a more 'apples to apples comparison' than comparing them at their respective long-range-cruise speeds. The G550 burns the additional fuel at mach .85 because its design isn't handling high speed drag as well as the G600/650.
Are you a car enthusiast and/or do you ever watch the BBC series Top Gear? They once did an entertaining segment where they "proved that a BMW M3 is more fuel efficient than the Toyota Prius". And they did prove it, the catch was in how they compared them: the Prius was driven as fast as possible around a race track, and the M3 merely had to keep up. Lo and behold, the Prius being driven as fast as it could used much more fuel than the M3 keeping up, which wasn't coming close to using all of its available power. The M3 was designed to drive at those speeds (or faster), the Prius wasn't.
Similarly, engineers' understanding of aerodynamics wasn't as good back when the G-V was developed as it is today. They can design a plane that can fly faster while incurring less drag. And we see this in seeing that long-range-cruise speeds (which correct me if I'm wrong, are defined as "the fastest speed the plane can fly where flying any slower would be no more efficient") on new models increasing. Commercial airliners have followed the same trend in long-range-cruise speed increases.
But you are a professional Gulfstream pilot and your employer is an early adopter of high end Gulfstream products, so I am sure that you are much more keyed-in to whatever they are doing than I am.
It just doesn't make sense to me, how unfavorably it seems to compare to its own stablemate G500 or even the older G550. The most damning comparison of all is probably against your own ride, the G650, where you can have your "apples to apples same speed" comparison and the smaller, lighter, shorter ranged G600 hardly saves any fuel. In fact, I think it would be interesting to do your "apples to apples comparison at the same speed AND range" and see how much fuel the G650 uses to fly 6,200nm @M0.85. Would the G650 actually use LESS fuel than the G600?