Magnesium is lighter, but brittle and very expensive. Hit the average pot hole in the UK and a magnesium wheel would break-up, where a Ford OE or aftermarket would probably buckle.
Less metal often means weaker, this is not always the case. Who knows the make up of the OEM alloy material and the aftermarket alloy materials used. I guess the aftermarket has a better grade of alloy used as their are bigger £££ margins to be made. As Ford would want to pay around £320 a set for the forged rims.
As for the OEM wheels I agree they are a great looking factory rim with absolutely nothing wrong with them.
As the Focus Mk3 is a world car and the Forged wheels are available worldwide they have to survive around the globe so would be engineered for that purpose going down a desert dust road in Australia or on a German Autobahn. Ford have to design/specify a compromise/cost.
RS Edition.
FPM375 Rocks
The rest of the “M” maps suck