Any restrictions cause the water to gently back up as it waits its turn to exit the tub spout.
Why no pex for tub spout drop.
There is no way to secure pex adequately to keep a tub spout from flopping around.
So it s fine to have 1 2 pex going to the shower head.
Copper keeps the spout in place.
Pex when you pull up on the diverter the entire spout moves.
You need an unrestricted flow out through the tub spout.
There is no stacking issue with the.
Valve drop to spout tube is within the 6 11 inch tolerance.
Eventually the line reaches the shower head and one gallon of water will exit there for every 4 or 5 that comes out the tub spout.
Valve to tub spout tube is.
If the valve has a built in diverter three handle valve this would not be an issue.
The tub spout can be copper brass or galvanized pipe.
You cannot use pex because the pipe coming out of the valve needs to be able to support the weight of the tub spout and handle the abuse of pulling pushing down of the divirter built into the spout.
Shark bite copper shark bite 90 iron pipe pex everywhere else.
The interior diameter of pex is actually a fraction smaller than copper this forces water to come out of the shower at the same time as the tub spout.
With a built in diverter you could use a pex drop eared ninety degree female adapter and a nipple but that is more trouble than it is worth.