I would skip carbonating a keg with sugar. I wouldn't want the additional sediment/trub in the bottom of the keg that will cause. Besides, depending on the recipe, yeast, etc. you could have sugar left that the yeast was just too worn out to eat. IF you used a scale to measure the priming sugar, I hope you used the amount for keg, not bottles.
Did you taste a sample of the brew (when you took the FG) before kegging it?? How long was it in fermenter before you moved it to keg?
BTW, what are buttering hops?