I plan on eventually upgrading my mainsheet, the stock 5/16" fuzzy dacron is not that smooth thru the blocks. In light winds, it almost seems to fat to flow easily. Definitely would not go any larger.
I will likely go to salsa, probably 1/4", just for flowability thru the blocks. I also have a ratchet upper block. This may be the best upgrade I made yet. It is switchable, so in light air, you can turn it off, when it kicks up a little, switch it on and it takes very little effort to hold. Harken makes this upper block, HSB411, available by special order only. See this thread
http://www.catsailor.com/forums/showflat.php?Cat=0&Number=35616&an=0&page=1#Post35616 I think catsailor sells them. Contact Rick White. You don't need the whole system, just the upper. The stock lower fiddle works fine w/ a double upper.
Another good option, but more pricey, is the lower fiddle w/ ratchet
https://store.catsailor.com/ProductCart/pc/viewPrd.asp?idcategory=0&idproduct=9243 like this.
As for the sheet falling thru the lacing, I don't have that problem siince I have the SE tramp, but I think the Hobie catalog has something called the mainsheet overboard preventer, or you could make something out of a strip of sunbrella canvas from an boat-top shop, just weave it thru the lacing.