Didn't a fairly recent Airbus study indicate that a significant amount of noise on the approach is generated aerodynamically - maybe even equalling the amount of noise generated by the engines?
In that case, and anyway recognising that late on in the approach when dirtied up the engines run at higher settings, that means that it is the people living near the bottom of the ILS who get the worst hit.
In that case, surely this idea would give a significant improvement just where it is needed most and has to be worth serious consideration. Very few of the aeroplanes landing at LHR are likely to be at max landing mass/weight and even if they were I would expect a 747-400 at max landing mass to use less runway to land than the same aircraft uses to get airborne at max take off mass.
Meanwhile, Airbus (and hopefully Boeing) are looking at reducing the noise generated by landing gear, flaps, undercarriage doors, brake hydraulic pipes etc. Less aerodynamic noise = less drag = lower power settings = even less noise.