Do Gulfstream have a history of underestimating and then revising performance figures to maintain the backlog of their older products?
Kind of. With the G650/ER they originally said that it was going to have a 5,000nm M .90 range, but then after they launched they said, "oops it actually has a 6,000nm M.90 range". And then there was the fact that 1-2 years, after having marketed the G650 as "being about the same range as the G550 because we found that's what our customers want", they released the ER variant which just enables another fuel tank that already existed in the base model, good for 500nm of additional range.