Well... I guess the fact that there have been a succession of marques from GII through to G650 suggests that the customers are pretty happy. Notwithstanding the aerodynamic questions you raise, that would seem the most important thing when it comes to measuring the success of a brand.
IMHO, any wing/airframe/engine combination needs to have a focus - if you try to do too much with one aircraft you run the risk of creating a jack of all trades, but master of none. Perhaps there have been tradeoffs to ensure performance in particular areas, at the expense of others.
Who knows. Maybe the -750 will have leading edge slats, or some other design feature previously unheard of on Gulfstreams. No doubt it will have 9 cabin windows so large that pax will be freaked out by the fishbowl view in the cruise.
I can remember a time when Dassault sold Falcons without winglets because (so they said) their wings were better designed, and didn't need them. Viva change and innovation...