My math comes to about half that on the weight hit:

12 extra spokes and nips at 7 grams each = 84
500g (ultegra hubs) - 350 (WI) = 150
Total difference = 234g.

My weights may be slightly off because I didn't look them up, but what else am I missing?

Also have you looked at the 23mm offerings from kinlin, h-plus or pascenti? Many people have found them to be nicer to build with.