"I have read the 5V supply will not be enough as it could lead to the unit becoming unstable."That seems wrong. If your 5 Volt supply is well-regulated and can supply the current, it should work fine. But you don't use the barrel connector with 5 Volt power. It goes directly to the board.
I just wanted to be on the safe side and go with the 15 or come up with a voltage divider to bring it down to the recomended levels.