Guy,
What am I missing? If the lead traffic, worst case, instantly slows to 140 @ 5 miles, as it touches down the following fraffic is still @ 6 miles doing 160 if the lead is a 380. As you said above, the separation will reduce by approx 1 mile. Surely this is built into your inital spacing.
Alternatively, when you get your IAS readout, let's see who slows early!