Don't go to immigration, as this is an airside transfer -- stay airside! There is a security check from arrivals to departures but that typically takes under 5 min even at busy times. For ontime arrival you will be fine, unless TG has some nasty rule requiring check-in 2 hours prior to departure time ... which quite a few airlines now do for O/D discount tickets. Online check-in would be a necessity if that is the case. I'd suggest phoning TG and have them link your flight into HKG so they know you have a connection and not originating in HKG!!!