Hi Dave,
I don't have the exact weight of the OEM 1999 NSX exhaust but my 1994 OEM exhaust weighs 19.7 kg or 43.4 lbs and that is the weight of my own exhaust on my scale.
For comparison, I have replaced it with the Taitec GTLW and that weighs 18.3 lbs (8.3 kg) in comparison.