I've transferred a few times at Hong Kong and each time it has been painless with little to no queues at all to contend with getting through security (admittedly these have all been at about 9pm so I'm not sure what it will be like earlier in the afternoon). Both Cathay First lounges (The Wing and The Pier) are excellent, the Pier as mentioned is a bit of a trek to get to from where the BA flight arrives into and isn't close to where Qantas will depart from either.
If time is short then go to the Wing as you'll get about 30 minutes extra lounge time (depending on your walking speed). If like me though you like to stretch your legs between flights then the head to the Pier.