Sorry to bang on about this but talking to professionals in ATC they say this will make little difference ...
Problem is one of capacity in the air, whether thats mixed mode on 2 or even 3 runways..........
Not sure who you're talking to, but NATS would appear to disagree with you, having done detailed studies of different configurations for the 3rd runway and come up with two possible configurations capable of supporting 130 movements an hour.
Department for Transport - Technical reports