There is so much that could be done to improve this which would cost nothing and make better use of the resources already available...

If the shuttle trains were also made to run in "Mixed Mode" with one half of each train as Arrivals and the other half as Departures this would also help.

The already discussed general opening of the foot tunnels too.

This should allow the possibliity to make your transfer in the Terminal where you land (5A, B or C) irresptive of where your next flight leaves, and then make your way to the next flight, hence spreading the load from the already bursting T5A Security.

The execption this is perhaps INT - DOM transfers who have to go through passport control which is currently only situated at T5A.

Other advantages would be the freedom to "roam" to the other statelites, and use the shops and lounges there hence again spreading the load from the what feels like an overloaded T5A.