If you do it, make sure to taper the extremities properly so the join is smooth. Last week end at Rutland the leading F18 had to come back ashore without finishing the first race because their main sheet broke right at the taper. The transition was really abrupt. From your pictures Dazz I'd say the outer is not tapered enough inside the core in yours.

And I agree with Dazz, nothing in the F18 box rule limits your main purchase. However more than 10:1 without a cascade would be quite a lot of rope around...