A disadvantage of SPG up is extra chest clutter. You see it at the end of that video with the two SPGs touching, or overlapping for me. I've had them get stuck inside the necklace bungee, or in chest D rings. Or form a stack with the inflator and necklace reg all under the reg you're breathing. Which needs to be shifted aside to look back. That isn't the SPGs fault, but their getting stuck in it doesn't help.
My tanks seem back and under my arm properly but might not be, or rotated just right, and 4" hoses might help. But managing clutter is a factor. Having them hanging way out in space increases the odds of whacking something delicate with them. I'm still playing with 6" SPGs up.
9" SPG down had cleaner tank positioning and handling. Transmitters would be nice.