Lowering the water pressure in the bathtub spout

According to the installation manual for the spout, the nipple for the spout should be installed a min of 3/8" to a max of 3/4" past the finished wall. There is a pipe adapter that you screw onto the nipple. The spout slides over this adaptor and is set in place with a set screw located on the bottom. This allows for the correct orientation of the spout.

The hole for the spout through the tile and back board should be about 1/4" to 1/2" larger than the nipple diameter allowing for an 1/8" to 1/4" gap between the nipple and the tile.

There should be installation instructions for the diverter valve that will include the vertical location of the valve relative to the spout, as well as the set back distance in the wall. There is usually a plastic over on the diverter valve that is to be set flush with the finished wall. This cover also provides the opening required for the valve. Check the diverter valve instructions.

Hardly any noticeable difference; though a "high flow" showerhead would make any difference WORSE, not better, as well as increasing your water use needlessly.

The water is already traveling through many feet of pipe to get to the bathroom. Whether there's 4 more feet of pipe to the showerhead or 9 more feet of pipe to the showerhead makes very little difference, particularly at the typical 2.5 GPM flow. At a more efficient 1.5 GPM flow, even less.

4 feet of 1/2" plastic pipe @2.5 gpm = 0.625 feet added head. (0.27 PSI)

9 feet of 1/2" plastic pipe @2.5 gpm = 1.406 feet added head. (0.61 PSI)

4 feet of 1/2" plastic pipe @1.5 gpm = 0.243 feet added head. (0.11 PSI)

9 feet of 1/2" plastic pipe @1.5 gpm = 0.547 feet added head. (0.23 PSI)

There's also no particular reason to put the valve set closer to the spout - so you could just put the longer pipe on the spout, and take advantage of the pipes in the shower wall.