Happens that I answered this in another thread.
My 172 was in for warranty work and the dealer loaned me a Megane. I weighed the Megane's spare wheel, then when I got my 172 back I weighed its spare wheel. The Megane's 16" steel wheel with 205 tyre only weighed 1kg more than the 172's 15" mag wheel with 195 tyre.
There is no doubt that you can make a lightweight mag wheel, and some are. But the reality is that car manufacturers put mag wheels on cars for style, not for lightness, and they require them to be durable. So what they supply standard is very little if anything lighter than the equivalent sized steel wheel.