To me it would make much more sense to utilise the platforms which already exist at T5 and put in a westward tunnel down to the GWML to connect east of Langley station, where there is plenty of room for a fly-under and /or tunnel portals. This would reduce conflicting movements between trains within the airport network as trains can divert via Heathrow rather than travel via Slough.
I agree, I hope that is the plan!
BAA has made considerable efforts in getting airport workers to use public transport to the airport (including discounted ticket options exclusive to staff, arranging for additional early morning / late night buses into the airport to meet staff starting times). Heathrow Connect is popular with airport staff and many of the early morning and late evening services resemble staff trains as 75% of the passengers are airport workers. I would expect the same sort of thing to apply to any extensions to the airport's transport links in future.
If there are so many workers using Heathrow Connect and night buses to get to and from the airport, how many will be attracted from their cars on to the new service? I still think that the airport staff travel is not a strong argument!