On at least 3 occasions in my flying career I have had all passengers, cabin baggage, and hold baggage actually weighed. In every case the actual total weight was less than would have been calculated using assumed weights.
On the type I fly (typical) an extra 500 kgs changes take off speeds by 1 (ONE) knot, although of course extra weight would affect performance. As has been stated previously Gross Performance is downgraded to Net and wind components (head and tail) are factored.