The race package stock is 16:1 through a cascade (blocks on the clew + clam cleats on the mast by where the diamond wires enter)... without the cascade you only have 8:1.
16:1 is definitely enough, we've ripped our main with it! And it was less than a season old.
Check out the Nacra Assembly Guide online, P.18 i'm sure you can find a picture of it.
And the downhaul runs a lot better with a small line (4mm). We are running 4mm FSE Robline Dinghy control and it runs very well and uncleats from the wire everytime no matter how much pressure.