Don,
Not sure whether it is linked to the huge area of drag surfaces available or other factors come into play but the 380 loses speed much more quickly than just about everything else that flies in here (even the 330s), after the speed limitation expires. Our radar only shows groundspeed at the mo and it usually shows them grounding at 135 kts or so - often already at 5 miles out, where almost everything else is still showing 160-170. That's half a mile a minute of difference, which would already give at least 1 mile of reduction on final.
Experience has shown that spacing continues to reduce and, short of asking the following traffic to fly 140kts 8-9 miles out (which I personally hate to do - others don't) or go-around, the only other way of ensuring adequate spacing when the 380 touches down (as we are required to do - there is NO leeway) is to provide it when the second a/c turns inbound. I personally always use 2 miles extra - depending on the winds, this always reduces by more than a mile and often by the full two.