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..........
Real answer is to close Stansted Gatwick etc and put 3 more runways /terminals into Heathrow!
...it will however create 000s of jobs and handily fill a big hole in the construction/housing industry at the moment which is bleeding jobs !