My opinion and prediction:
1. Porsche will reduce emissions to a significant degree in various ways (including Cayenne and Panamera hybrids, DFI etc). Obviously, this will increase the production cost per unit(by some 3000-3500 euros per car I've read somewhere). Some of the extra cost will have to absorbed by the company thus reducing profitability to an extent. However, whatever they do will not get them down to the 130gr/km level by 2012.
2. One way or other the Porsche CO2 will be taken into the calculation of the range average of the VW Group, because by that time Porsche will be more integrated with VW. The same is already true of other luxury marques like Aston Martin (Ford) Ferrari (FIAT) Bentley and Lambo (VW) whose CO2 is not viewed now in isolation.
3. Finally, if there will be an across the range residual above the 130gr/km norm the car ( the 120 was revised ) companies will have to pay penalties which will enrich the EU coffers but I don't think will be set a levels that will be crippling for the industry, right away at least. Thousands of jobs rely on the motor industry in Europe and EU is based on compromise because there is no other practical way to run this heterogeneous Union.

Finally, I don't think someone will not buy a 998 Turbo or similar car in 2012 because it will cost 5000,6000 or even 10000 euros more than it does now.