Assuming you've not done HK before then that seems like a reasonable plan if the weather is OK (which it probably will be at this time of year). If the visibility is good and you find yourself with time and would like to stretch your legs then the path that goes around Victoria Peak from the Peak Galleria is about 5km in total and is flat and easy walking so allow one hour if you are reasonably fit and don't take too long over photos of the excellent views.

For timing, you need to get a train by around 2pm for a 3:15pm flight (currently the interval is 12 mins) - the full timetable is here: http://www.mtrcorp.com/eng/train/ae_timetable_e.htm so you need to get the 2pm train if you expect to more or less go straight to the gate.

If you want to be sure of some time in the First class part of The Wing (as an EXP) then go a little earlier. The food there is better than onboard J (and the wine is similar to onboard J, but more choice), so I would probably aim for the 13:24 train to leave enough time to take a slightly late lunch there.