There will also be a cost reduction from the more efficient use of the ARC (anti-reflective coating), top coats, and Photoresist applications on the larger wafers. Coat dispense volumes do not go up significantly with larger wafers in a spin coat application so you effectively get more imaging for the same volume of chemical. Seeing as many of the lithography materials are some of the most expensive in the process this benefit can be very significant. Of course controlling these thicknesses to within a few angstroms across essentially a medium pizza will be a major challenge.
Also I was pretty sure the SEMI standard was for 450mm wafers. It will be interesting to see how many people adopt the new equipment for 450mm production, because the up front costs will be astronomical.