So yeah, I guess the fact that you can put boxes on boxes is known by now, but it seems like I can stack every factory item on top of each other. So I thought "Maybe I could speed up the conveyor belts by putting them on each other!" and tried. And... it worked.
- The top line was normal (1 conveyor belt per block)
- The middle line was 2 conveyor belts per block
- The lower line was 13 conveyor belts per block
So yeah, you can just build item cannons by stacking conveyor belts on top of each other. I think the stacking bug should be fixed someday. But still, it's fun!