Well, it's conceivable you'll have no problems at all either, so who knows. It's your tank and if you think you'll be happy with that arrangement then that's really all that matters.
One parting thought though, it's not so much the diameter of the pipe that is causing the pressure loss, instead it's the spring that's pushing the flapper valve back (so that in the event of a loss of pressure, it's closing back and hopefully creating that seal). Also watch out for that spring, I don't know what they're made of, but since it's in saltwater watch out for corrosion. Luckily the unions will make it easy to periodically inspect for that.
How come you don't like the idea of a closed loop? Just curious.. If it were me that's probably what I would have looked at.
And .. not that it matters now but another idea might have been to go with loc-line and bring the end of the nozzle up closer to the water surface. Would help with surface agitation and if the nozzle was high enough then the water surface wouldn't have far to drop in a power-off situation.
But anyhow .. anything that works in the end is good enough. Good luck!