This has been stated and confirmed several times.  

A suggestion:   If you have a power supply with good regulation, no load to rated load, and you use power leads with very low resistance between the power supply and radio, you can run the power supply at 14.5 volts.  This will certainly improve the transmit IMD. 

Power leads should be of minimum length required, use #10 AWG wire, crimp on the correct terminals required for each end.   If in doubt crimp and solder terminals.  AND do connect the radio power cable direct to the power supply.  DO NOT use a DC power distribution strip.  Every connection adds resistance and results in voltage drop.  

73

Bob, K4TAX