Hi Vince,
Well - it's not producing anything like 600bhp until you get up to around 4,500 rpm or whatever, and under load. Sitting there idling in traffic (which is the big problem time!) it'll only be producing a small fraction of that. The real test of any cooling system is when you have a decent, long motorway cruise at high speed then suddenly need to idle for an hour, like joining the queue for the Dartford crossing!
I don't think I'd be too concerned if it was stabilising at around 80°C, although a little higher would be nicer. Some folk run motors down at 60-65°C which is asking for trouble in my opinion. Sounds as if just upping the fan switch range a bit to allow it to cut out now and then is a good place to start.