Thanks Scone,
I'm truly surprised by that - I can't understand how you can extract more from the engine without it producing more CO2 from the rear.
Even if it is more, there is a chance it might work out the same given the kit is fitted at the dealers, and V11D (think thats the correct form?!) is based on the emissions as it leaves the factory.
Having said that, after adding options to both choices of works tune, I need to decide if the factory option is worth the extra £45 per month in tax vs a similarly extra'd up stage 1.