DRUK,
Without sounding facetious, the answer to your question is 'it depends', which was my point.
Yes, on paper, if you assume a schedule of 40-42 departures an hour, you'd think that one or two A380s arriving per hour would not have much effect, and I could quote the average time lost on the departure runway for an A380 arrival, but rarely are things identical to the average at LHR.