Well I guess it's down to the quality of the ground power. Having operated without an APU, with ground power supplied to both forward external AC connections the RH engine was started without too much drama. The EFB took a few minutes to become fully operational but apart from that there was no delay in getting going. Starting the LH engine from the RH engine generators appeared to be slightly quicker than using the APU generators but then again when using the APU we tend to start both engines together. At one point it did look as though we would have to use a single source of ground power for engine start. The engineers opinion was that this would work although it would be extremely slow and with the load-shedding it would take longer for all the systems to come back online.
Using the aft AC connector is a bit of a non-starter. A panel has to be removed to gain access and then re-fitted so a qualified engineer is required. Also probably not a nice job with the engine running.
The main problem with no APU is keeping the cabin cool especially with no ground conditioned air. Therefore, using the Ground Test Enable to run one pack from external power would be most useful. Obviously this does place extra load on the ground power source and it would be a disaster if it tripped off line. However, if it is going to be used to start an engine perhaps it would be better to know earlier rather than later if it was not up to the job.