I would steer clear of cast aluminum wheels. They will be heavier and as the other op stated, will likely not be able to carry the same load.
17 inch forged Alcoa wheels will hold more weight per wheel than the current steel wheels and the net weight gain should be negligible. A forged aluminum wheel will drop unsprung weight when holding the same size and will normally allow you to go up one wheel size without a large weight penalty and a reduction in CCC. Every pound you add in unsprung wheel weight will drop your CCC by 4 pounds. Aluminum wheels have been reported to give a smoother ride thanks to their trueness. I looked at a wheel swap myself, sticking with the same size wheel. A same size swap would net an 85 lb drop in unsprung weight. Going up an inch in size added a few pounds per wheel but the added material in the tires needed to carry the same weight would cause another bump in unsprung weight for the aluminum wheel / tire combo. The replacement inner steel wheels will add some weight on top of the added rubber weight. IMO increasing the rim size just wasn't worth the potential loss in 100 - 200 lbs of CCC.
If you're planning on upping the brakes, spring packs, axles etc to punch up the CCC capacity to the old E550 stats then the rim swap would make some sense. Of course that would be the cheap part of the work at that point
The major down sides to a forged aluminum wheel are:
A lack of wheel selection
Cost (1200 - 1600 for a set of 4 16" wheels)
Added thickness of the rims prevents using 4 on the back due to a lack of wheel stud length, wheel well
Aluminum tends to fracture instead of bend. If you smack a curb hard enough to bend a steel wheel you might end up destroying the aluminum wheel. The steel wheel can be repaired, the aluminum wheel will likely need to be replaced.