I really hate to be a KILL-JOY
but... I have done this exact mod on a Honda VT750 Shadow. It DID increase the sound to a louder, low end rumble, BUT....
3 mechanics REFUSED outright to provide Vic RoadWorthy certificate as they MEASURED the db
and knew I had hole punched the baffle, even though it is not a visible mod....
its the oldest trick in the book!
The only way to get roadworthy for selling was to WELD a restriction plate over the baffle area, which requires removal of the muffler from the bike (for the remote, but real risk of weld flash of piston to cylinder wall / head and / or frying the voltage regulator / ECU.)
Punching the baffle took 3 minutes, repairing the hole prior to sale took 2 hrs and $150....
BTW NO significant benefit was noticed in power at all on that bike and I doubt if any improvement would be noticed on the ST1300
IF you are planning to keep the bike for many years, I feel it is better to invest $600 in an aftermarket new, quality muffler (Akropovic, Staintune etc.) as they all have REMOVABLE decibal killers (silencers) and will pass ADR / Roadworthy...
my 2c, good luck