Thanks for the replies so far. It appears that there's no quick and easy answer, like in so many things in aviation.
Let's assume two targets on an exact opposite course at the same altitude. Both straight and level, no manouvers whatsoever. In the first case they both travel at 200kts, in the second case they both travel at 300kts. Would it be safe to assume that the TIME before conflict of a STCA alert would be identical (and therefore the distance between the targets at the moment of the alert is different)?