The problem is quite simply focal plane curvature. The light fixture's beam is part of a sphere, and without additional lens elements, the "focal plane" isn't a plane, it's a curved part of a smaller sphere. Here's a basic discussion of the problem. The only way to fix this is to add more elements to the optical path to compensate for this. This is one of the reasons that camera lenses have multiple elements, not just one or two, as is the case with both ETC and Chauvet leko tubes.
Metal gobos - at least those I have used, are almost useless. They warp significantly as they get warm, so the focus changes as the fixture heats up. If you get lucky and the metal gobo happens to bow out rather than in, it might help slightly as the gobo might be closer to the shape of the focal plane of the light, but that's not practical. If it bows inward toward the lamp, it will actually be MUCH worse than a glass gobo, as it's bowing in the opposite direction of the focal plane. It's theoretically possible there is some distortion from the glass of a glass gobo, but glass holds the gobo rigid under heat, unlike metal. Also, my movers use glass gobos and look great.
I used the gobos in the pictures above because I had two of them that were the same (other than being mirror images of each other). My original application (shown in the first post of this thread) was actually text. I have done other logos as well where having a sharp projection is very desirable.
