Mixed mode will not address congestion at LHR. Not only will departing planes have queue to take off as at present but will also have to wait for planes to land. Arriving aircraft will be circling for longer as they queue to land and this will be lengthend by the need to wait for planes taking off, but it will increase airfield capacity, by about 12%(?).
I will see if I can find a link to the models BUT mixed more does give more movements per hour as it's makes more efficient use of existing runway capacity. It allows peaks and troughs in the arrival and departure flows to be smoothed downwards. I have seen aircraft taxi from T3 to line up number one on 27R and depart and then ten minutes later, same scenario with about seven ahead at the holding point. At the moment the model is quite static due to the 0700-1500 and 1500-2300 runway allocations but in mixed mode, ATC have more dynamic ability to filter traffic to where the queues are smallest.
Heathrow on paper is full all the time but since nothing really runs to schedule all the time in reality, there are some pain points during the day, particularly with regard to departing heavies one after the other after the other which gets worse when everything is heading on the same SIDs.