I believe that 12/8 merely signifies a 6/8 tune which you play as fast as you can. It's arbitrariness, not mathematics, that says so.

Your right in the last part. In fact there is nothing that says that 12/8 tunes should be played any faster than 6/8 tunes. The difference is that 6/8 means two beats in a bar, 12/8 means four beats. The difference is in the emphasis not the tempo.

There is a good case for arguing that dotted hornpipes are in fact 12/8 tunes. Although they are written in 4/4 this is just convention. The way they are played is more like 12/8. To come back to the point, hornpipes generally should not be played too quickly.