Galvos are a type of motor, an inductive load. They also change direction all the time so they won't reach a steady state. The net result is that your galvos will be causing voltage spikes and EMI on the power lines which can affect your other devices. You don't want excess current or voltage spikes surging through other devices like laser diodes or your control circuitry.
Another thing Cyparagon pointed out is how much energy (as heat) you'll be wasting with all that extra voltage, especially if you're using a P3 directly. From my understanding, those P3s aren't switching supplies--which are efficient in converting between different voltages; rather I believe the P3 is uses a linear regulator, or a transistor. Those devices just "burn off" the excess voltage like putting a well-regulated resistor in line. It'd be like hooking up a DDL driver to 24V.
A 7V supply or something like that would reduce power and stress on your circuits. They're not that expensive anyway, and then you can separately power down different circuits as needed.
Also think about it this way: you've already put hundreds of dollars into your other equipment. Don't ruin them because you bought some cheapo power supplies or tried to force some of the devices to use too much voltage. Even those cheap Reke projectors use multiple power supplies in them.
