The air can't flow around the lip of the velocity stack very effectively because the walls are so close (and therefore so is most of the boundary layer) so the stacks can't 'gather' air very efficiently from all directions. You're only going to have good velocity around the center of the stack, not around the whole perimeter like you want. The radius profile on the lip on the stacks is very important in order to work well, so if you put that lip close to the other stacks and close to the wall, and you're not sitting in clean airflow, that lip might as well not be there and you could just as easily use a flat floor with a slight radius on the edge of the runners. There is a minimum spacing around the velocity stacks for them to actually have an advantage over a straight piece of tubing (of just a flat floor), it's usually 60-200% of their diameter, depending on the spacing above the plenum floor and the radius profile of the rolled lip.