Capacity is higher with continuous mixed mode at LHR
No argument there. But SGC's proposition implied that alternation, or absence of it, would affect capacity per se:
Without alternation and the night flight ban there is substantial capacity to unlock.
I think I'm right in saying that there is no difference in Heathrow's declared capacity whether it's on westerlies (with alternation) or easterlies (no alternation), within the current segregated mode regime.