DaveJ,
Perhaps this may work well when on a beam reach with the kite, but heading downwind, you really want to rotate it well out in front of the main to windward. There is no way you can do this if it is behind the forestay. If you are using a spin that was cut for your boat, then it most definitely will have a luff that is too loose as it is not being stretched out as much as it was designed to be. With a loose luff, unless it is way over trimmed, the sail will WANT to rotate into the forestay and get smushed out of shape.
Furthermore, a there is no way you can trim a spinnaker correctly if the luff is squished by the forestay as you want to constantly watch for that luff curl. I suggest that if you really want to put on some horsepower downwind, rig a bail to the masthead with a block for a new external halyard.
Just my $.02. YMMV