I don't know how much you are saving with a derailleur vs. the SA hub. I know the Nexus on mine isn't that heavy. If you compare the weight of the Nexus red label vs. the derailleur, gear cluster, extra chain and the actual hub itself and the longer spokes neded I don't think you will find there is a huge weight savings. I haven't weighed it but I bet it the difference isn't as much as you suspect. Most people just compare the weight of the internal hub vs. maybe the derailleur and cluster but you need to factor everything else in as well.

While I give up a bit in efficiency that is offset by where I ride and how dirty my chain used to get because of the low hanging derailleur.