I have now replaced R11 (now 5.6 Ohms) and R30 (now 30.8 Ohms), and squeezed the turns of L2 together more. The result has been most satisfying.
The power output is now as follows:
9V = 1.87W
12V = 3.14W
13.8V = 3.92W
9V = 1.66W
12V = 3.07W
13.8V = 3.98W
The rig is now performing as it should according to the specifications.