Andrew, the 6mm outer becomes a 4mm line without a core in it. have a 6 inch splice if you want, not going to hurt either way.
Jake, when you move onto a sail with 1 meter square top you need less travel to have the same effect. I agree 11:1 is too much, I find 10:1 is over the top sometimes.
I agree if you are sailing around a buoy course with refined upwind sailing. However, we often find ourselves (in distance racing) on a gonzo reach and the 10:1 is just too slow to get back in after a gonzo blast.
