Exhause goes out the blue pipe, all the way back towards the header via the black holes and then exits through the grey pipe. Reversing through the two black holes in the bulkheads is what gives the quiet aspect to the silencer. You can make those bigger as needed. The grey pipe is the one that needs to be enlarged for better exit and less containment or back pressure. That is the pipe you would want to replace with 7/16 or 11mm ID tube. You can experiment with the exact right size by making a blanking plate to replace the plate and long tube and drill a 11mm or 7/16 hole and see how she performs before getting and bending a new inner pipe.
Shortening the inner pipe you are replacing will also have an impact on performance as the route to get out is shorter, but also noisier.