Nice to hear from another 5.7 sailor. Warbird has the rihgt idea about prebend, make it match the sail, after you release old shrunken bolt rope at bottom of luff.
And yes, you definitely need more than stock 3:1 downhaul.
http://austinyachtclub.net/members/jimcasto/IMGP0316.JPG

Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis