Using -pipe is a big win. On a system with not enough RAM, and not using
   -pip, the stuff gets written to intermidiate files. If this is an MFS /tmp
   then this is swap, else it is ordinary file system. On a system with less
   memory and -pipe the processes get swapped. If the swapper/pager is good
   enough then what is the difference ?

Even if you scheduled optimally for this situation, you'd still have
to switch your working set 2*((N/PIPE_BUF)-1) times more, where N is
the amount of data output between stages.  And if you come up with
an optimal scheduler, I'd love to see it...