Sharing DMX512 GND with Power GND
DMX512 is electrically RS485, so you have the usual common mode range restrictions inherent to that standard (+12 -7V IIRC).

I would want to do some experiments before going there as performance will depend on just how noisy the loads are, some led drivers can be pretty horrible for PWM switching noise getting onto the supply cables.   

You will probably find that line termination helps, as does some LC filtering of the power at each node, but really you need to get 'the scope out and have a look.