I have traveled from the airport to Hk island more times than I can probably count. i have nevers een a taxi fare above 500 HKD, if someone was charged 800 HKD they were ripped off.
In terms of the rest, and I believe I posted it several years ago on thsi very thread, rule of thumb.
1 person traveling light. Take the train
2 people traveling light train is a possibility.
1 or more people with serious bags. Take a taxi
3 people traveling, take a taxi.
In terms of timing, it is all really six of one and half a dozen of another. If you are traveling light, manage to get to and on the train jsut as the doors are closing, and then when you arrive at the station, again you get on the shuttle just as it is about to leave, then you will have done by far the fastest possible way. If however, you wait 10-14 minutes for the train, and then wait again another 10-14 mintes for th shuttle, and are also lugging bags the whole way, you will in fact be slower than a taxi, even if there is traffic.