My apologies John, that was a rather flippant answer. Trying to be humorous rather than helpful.
I've not fished the bommies off Townsville but fishing for GTs in shallow, difficult water has some similarities regardless of the location. IMHO 30lb would be pushing it, even on small GTs. Fishing 30lb is one of my favourite things to do but getting even small GTs out of a metre or two of water in rocky or coral terrain is an expensive exercise. It's not a matter of drag, the 5000 is more than capable of stopping the run, it's just that with 50 metres of line out and maybe another half metre of flex in the rod tip there is just too much give in the system.
The fun, however, is in the trying. Time to practice that slim beauty until you can do it blindfolded.
Hope this helps.
Regards,
Trevor