The petcock passes more gas than the bike can burn under most conditions, that's why the floats are there to shut it off. Seems like bigger hose would have more available for quick delivery. But maybe I'm all wet.
Well, you are pretty damp. And you kind of answered your own question... Because fuel flow is gravity fed, in theory, the extra "weight" of more fuel in a fatter line would increase the pressure on the "drain" (that being the hole leading to the float bowl)
Think of a tall rain barrel full of water. You take a pistol and shoot a hole in the barrel right at the bottom, and also right near the top. The holes are the same size, but the water comming out of the bottom holes stream comes out with more force, the stream hitting the ground is farther away from the barrel than the impact point of the stream coming out of the upper hole.
You can see this with just punching two tiny holes in a 2 liter bottle , the bottom stream shoots out farther than the top one....so your thinking here does have merit.
But as you surmised, the difference between the the added weight of fuel in a 1/2 inch tube compared to a 5/16 tube would be fairly insignificant...but would be a moot issue as the valve in the float bowl meters the amount of fuel in the bowl and by design it never really "runs out" so the nano second you saved filling the bowl would never be felt. Not to mention...
That it is the weight of the gas in the tank
that makes the differnce here, not the weight of the fuel in the lines. So keeping your tank topped off would do more to keep fuel flowing quickly to the carbs.
The last thing that blows using that theory is the fuel in the lines can not flow any faster than the rate coming out of the petcock to replace it.
Even if you had 6 inch diameter fuel line, the tiny hole in the petcock would would only let fuel flow as fast as it can come out of that
It also brings up the issue that if you could increase the "pressure" of the fuel going to the carb enough , you could over-ride the float valve and flood the bike.