The water pipes for my house are mostly attached to the ceiling of my
basement (which makes it super easy to get to!). It starts out 3/4", and
gets reduced to 1/2". My shower valve has 1/2 threads on it, so I see the
need to reduce it at some point.

In the shower, the water flow seems to be diminished. One of my shower head
settings is some sort of water blast, where the water comes out of a half
dozen large ports. Both hot and cold just dribble. Other settings work OK,
but none of them give really good volume. I'm thinking that i'm the victim
of a water mizer shower valve, and that replacing it will fix the problem.
This is one of those valves where you have no control over volume - it's on
or off, and all you can do is adjust the temperature.

Question: Will 50 feet of 1/2 pipe reduce the flow of water such that I
won't get good volume at the shower head? Or should it work fine and I need
to look somewhere else for the problem (like the shower valve)?

I can be easily convinced to plumb my house with 3/4 pipe, but I'd like to
avoid drastic measures like that if not really necessary. The only place we
really need good volume is the shower, and if 1/2 pipe will give us that, I
don't need to start gutting my plumbing.