Thank you for your replies, I think as I understand it, I am quite happy but also looking forward to test this myself.
I was initially thinking how to factor in headwind, car weight, downhill slope and so on ... but in the end I thought, it is really quite simple:
No matter how much downhill, tailwind and what weight on board - if 1) the speed is INCREASING and 2) your foot is off the accelerator, then regen should do its best effort to keep that particular speed.
What I mean is, I hope that the Tesla S regen function is not just a "dumb" fixed resistance, but that it considers whether or not the car is actually accelerating, maintaining speed or decelerating.
Having to ride the (conventional) brakes on downhills in a modern electric car would be entirely disappointing, but it seems from the replies above (thanks) that this is NOT a problem.
I assure you that when I get my Tesla S eventually, I will test and report from the hills and mountain roads of Hong Kong