The whole point about stacking is a good one, and one I'm personally surprised that Heathrow haven't made more of in their proposals for a third runway.
By restricting operations to, for arguments' sake, 85% of capacity (with a third runway) they surely could argue that there would be far less aircraft circling over London and less fuel burnt. I'm not about to do the maths projections (I'll leave that to someone far more able than myself!) but seems that they have missed a sound environmental argument.
Of course it could be because they have absolutely no intention of limiting the capacity and will just ramp it right up towards 98% again I suppose...! (ahem, cynic!)