Hi Matt,
Have you tried to see what power options your radio gives when on a 13.8v external power supply? You should get all 4 options.
I have the 817 and 818, both give full power on a psu and external 12v battery, I don't use internal batteries.
Also the difference between 5 and 6w really isn't worth worrying about, to get the best out of these radios use good antennas and low loss coax.
Have fun?
Andy
M0RON