Hi Gonzo,
I was part of the Mixed Mode Working Group and we looked at the GMC operation in detail. It can be made to work as long as certain standard routeings are adhered to - these are not necessarily logical according to the routeings we use today. We considered choice of landing and departure runway by both Terminal geographical situation, e.g., T1 and T3 Northerly runway, T2 and T4 Southerly runway, and routeing criteria, i.e. Northbounds from the Northerly runway and Southbounds from the Southerly runway. We considered break out procedures for final approach, and SIDs with stepped climbs which could be flown independently from each runway with limited conflictions. Mixed mode can be made to work and there is a (slight) capacity benefit. Also don't forget with T5 there will be three GMC Controllers, so the workload even with mixed mode should be manageable. But I'll leave all that to you youngsters!