I've posted on this before, but I thought I'd take another crack at it. My head is swimming today, so forgive me if this turns out something less than transparent.
Relative vision (perception in general, but in this case, vision) is based on how far you can see in comparison to how fast you can act. For example, even though you can see just as far (or farther) in a car as opposed to on foot, your relative vision in a car is much less because you're moving so much faster.
Similarly, in a shmup, you're firing at the top of the screen (or at the side, in a side-scrolling shmup, but it's the same basic principle). You cannot fire sideways, so the only way to get an effective fire solution is to move from side to side. Therefore, your relative vision is measured in terms of how long it takes to maneuver into firing position after you've seen something.
This is why so many vertical shmups have narrowed screens instead of taking full "advantage" of the wider dimensions of your monitor. It simply takes too long to move into a useful firing position: your relative vision is too short to allow you to kill the enemies on the other side of the screen.
Similarly, this is why wide-area weapons such as homing missiles and spread shots are so powerful: they increase your "effective radius" massively - often by 50% of the screen width at maximum range. Since it takes less time to lock a solution onto an enemy, your relative vision is dramatically increased even though you can't actually see any more than before.
The Machine City allows you to fire in any direction, rendering the whole debate moot. It also renders moot the tension that comes with firing being closely linked to position. I'll have to be very careful about that...
On the upside, it also means that "wide-area" weapons are vastly less powerful than their cousins in normal shooters. Easier to balance.