There are several OEM aluminum wheels, I have one set that has solid spokes, I think forged, and one set that has the back of the spokes shaped like a bowl (those are obviously lighter) which I believe are cast.
My truck came from the factory with Steel Wheels, they weigh 42lbs and the OEM 265/70R17 LTX's weigh 48lbs so my OEM weight was 90 lbs.
I swapped to OEM forged aluminum wheels and best I can tell they are 24.5 lbs, with my 255/80R17 KM2's at 54lbs for a total weight of 78.5lbs.
I also run the cast alum OEM wheels with OEM tires for my summer setup, those wheels are 28.2lbs for a total of 76.2lbs.
The new setup will be 45lbs for the wheel and 79 for the tire, for a total of 124lbs. A 37% increase over stock, and a 57% increase over what I run now.
Nearly all accounts are that the rolling resistance is enough lower that the increase in unsprung weight doesn't decrease economy, and generally mileage improves.
As far as the weight on the wheel bearings, and ball joints I don't think it's an issue either. The balljoints are already Dynatrac's, and the front wheel bearings are a bit bigger with the Spin Free, and spaced better to handle the weight. In doing some research it seems the bearings used on the Yukon kit are also used on the rear of some F-450's...