It is very apparent when you run the vertical thruster at any power level that it is not as powerful as the 2 main thrusters. The sound of the thrusters alone at the different power levels is enough audible evidence. Since the stock Afro12 ESC does not need calibration, how does one increase the apparent power of the vertical thruster?
What version of the software are you using James? A long time back we did have some asymmetry. In the current software, forward and backwards should have the same power profile as up and down with the vertical thruster. But do note that the yaw port/starboard is not power limited with the same formula as the rest of the directions.