10 speed DSG will be more fuel/CO2 efficient for sure, however the top gear will always be set to whatever they consider to be the "typical maximum cruising speed" at whatever the most fuel efficient RPM value is (probably sub-boost RPM - I seem to remember that typically 70mph is about 1.5k RPM in 6th - thats probably around the sub-boost rev limit).
It seems unlikely that say 8th gear will be for 70mph and 9th and 10th are for effeciency at 80 and 90mph (for example)
What I would hope from 10 speed DSG though is that DSG will return the same CO2 figure as manual in the "approved test"... and thats all VAG actually care about!