Yep it really is ridiculous isn't it, and some call this a developed country.........
It is wheels that is the main contributor to changing CO2 levels as far as optional spec is concerned, which is why I quoted above using the example of a 530 Sport, so it has the wider 18" wheels fitted. The rest of the spec doesn't really make much of a difference, hence why a base IS250 auto has the same emissions as an IS250 SE-L with all the toys.........
As far as Mercedes is concerned, if you choose optional alloys, the CO2 emissions on your V5 is actually dependent on the wheels you choose, so you don't get away with it that way either (as an example, my dad's CLS has a higher CO2 emissions rating on the V5 because he has the 9.5" rear wheels)