It is worth pointing out that:
a) Reverse thrust is limited to a percentage of maximum N1/EPR;
b) Most modern fan engines don't reverse the 'hot' air from the core, only the 'cold' air from the fan; and
c) The exhaust plume is angled forwards at about 45°, so that only about 70% of the thrust is effective in decelleration.
If we assume that the factor in a) is 80%, that in b) is 66% and that in c) is 70%, we can see that 'full reverse' for a 23.5K CFM engine is only about 8.7K or 37% of the total forward thrust available!
I realise that this does not explain the change in retardation noticed at 80kts, but it shows how relatively puny the reversers are. Still like to have them on a wet/icy runway though.