I will be looking for a *plug* for the mast track for the mainsail. I'm going to call Catalina Direct to see if they have a device for that purpose. My C-18 has a small genny-type jib (I think 130%?). It is easy to handle w/ the roller furling. I sail in lakes, and the jib seems to perform well.
I have a track stop for this purpose. It is cylynder about 1 inch long and has a tightening nut and nylon washers. It works well and I got it from my local marine store, he through it in for free with some other items I bought. I have also seen this item at West Marine.
I bought an oversized Genoa (~165%) from a second-hand sail loft. It reduced the performasnce in close-reach conditions but the increase in speed is absolutely incredible. the standard working jib is grossely inadequate in medium to light winds (1-5 in beaufort scale), so ANY oversized sail will be welcome. In addition, the 18 is so stable that you do not compromise your safety. By the way, I sail on Chesapeake, and we have NO wind from June through August.