If you have your AADSAS app ready and you are interested then you should. GPA is important, but perhaps you can impress them with other aspects of your application.
To answer your last question, Columbia seems to be relatively prompt. I can only speak from experience, but I was fortunate enough to hear back from Columbia pretty quickly and interview dates were available in November. Or maybe I just happened to submit my application before they sent out a wave of interview invites. Don't know but I was surprised with the turnaround.