Its an interesting way of calculating it.
I was think more at an empirical level in that in terms of perfect combustion ( CH2+ 3O = H2O+CO2) the CCGT is more efficient as it does not produce as many nasties because they are turned into CO2 very effectively ( instead of say CO). Whereas a diesel engine isn't as good at achieving good combustion.
Your losses for the transmission etc are very conservative and does it consider the efficiency of the electric motor in the car.
Interesting stuff, but my main point was to point out that in terms of CO2 emissions it's not all great news for electric cars, like I say, unless the electricity comes from a technology that does not produce CO2