The weight sounds about right, very popular rim on guys owning Skylines too. Yours do look a bit different the the model 6's I know, the 5 wheel stud holes are usually a lot more recessed into the outer hub (Just looked at your first post, those are the ones).
Before deciding anything, actually weigh yours :-)
In terms of being one of the lightest, you do get much light, 57c's for example. The lightest I know of however is the KAD F1's - Forged Magnesium - 2.1kg without the rubber.
They look like this:

