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.