Mark,
I have a 4.3 m Stessl tinny (actually 4.5 m LOA) with a 40 hp. It is an old 'heavy duty' tinny which has been converted to a centre console, and has plenty of freeboard and beam.
I go offshore regularly no problems. Watch the swell size and pick your days. 15 knots offshore is no worse than 15 knots on the bay, unless there is a swell running. It does depend on your experience and skill level with your boat, but really you should have no problems if you are confident on the bay.
Go out from Mooloolaba or the Gold Coast seaway as there is no dangerous bar to cross (unless there is a swell of 2 m or more running). Out from Mooloolaba the Gneerings and Murphies reefs are around 12 km. From the Gold Coast, there are the 12, 18 and 24 fathom reefs, which are also pretty close in.
Ideally, team up with another boat so you can follow them out to a good spot and for safety.
Jeremy