The only issues I'm having with the main sail is the jib. Since an 18 runs a genoa which extends beyond the mast it has the effect of backwinding the main at the luff. I have a regular 18 jib, not the taller SX. Can't decide now whether to buy a SX jib or convert to a self tacker and run a Tiger jib. It would be nice to remove the jib blocks and track and have everything at the front crossbar. The self tacker is pricey so I don't know. It would also effect my portsmouth rating, effectively making it a Tiger by the numbers. Maybe for X-mas.
A flat cut jib like the Tiger works well on a H18SX when coupled with a Tiger main or similar. One issue is the sheeting angle - when a friend of mine was testing this set up we had to run the Tiger jib higher up the forestay (just an inch or two to get a nice angle.
The boat will point better and will be less over powered when reaching. With the spinnaker you don't need the big jib downwind. It just backwinds the main and messes with the air flow from the spinnaker - particulally if you run with a short spin pole like Hobie suggest in their setup instructions.