Certainly if you're considering a 99, the rated power on electricity is much higher than on diesel, although the starting TE should be the same. AC it'll accelerate better through the whole speed range, and having it already pan up whilst waiting for the road should give it better all round performance.
If power changeovers are considered a performance risk (and they obviously are more of a risk than not changing over) then wiring the arrivals road far enough that the train can clear the main line on electric would make sense too, so any issues can be dealt with out of the way.
In the case of London Gateway, there are no real obvious complicating factors to getting the wires up, so there'd be a matter of once you start the project to wire the first 500m, you might as well carry on to the end, which is the kind of scope creep that'll result in nothing getting done...
If power changeovers are considered a performance risk (and they obviously are more of a risk than not changing over) then wiring the arrivals road far enough that the train can clear the main line on electric would make sense too, so any issues can be dealt with out of the way.
In the case of London Gateway, there are no real obvious complicating factors to getting the wires up, so there'd be a matter of once you start the project to wire the first 500m, you might as well carry on to the end, which is the kind of scope creep that'll result in nothing getting done...
