Matt,

The F18 chute is certainly a higher aspect ratio than the Hobie spinnaker for the TheMightyHobie18 and should easily sheet to just in front of the shrouds. You should get a luff and hoist measurement before planning the modification. On the F18, the spinnaker attaches very high on the mast a little over 2/3 the distance from the hound to the top - this could be too high for a Hobie 18 or at the very least, leave you with a setup that you have to be careful to maintain mainsheet tension on to support the mast (we have to be careful on the Nacra F18). You could probably lower the spinnaker pole to compensate for the extra hoist height. I don't think an F18 spinnaker is going to be any more powerful than anything else because the trend in spinnaker design is getting flatter and flatter...but hoist height could be problematic.

Secondly, your attachment point will work to a tang or other fixed position on the mast - but you will have difficulty achieving proper mast rotation because the spinnaker will want to pull the mast forward. Most current systems allow the spinnaker hoist point to rotate freely around the mast so that it doesn't affect mast rotation.

Wouter has a couple of web pages (and a Word document) that does good job explaining several ways to rig the upper end of the spinnaker - but I don't have the link handy.