Both sails want to be somewhat curvy to make power. The main should be twisted off, just enought to notice, to account for wind shear and the higher velocities at masthead levels. I have the best luck dropping the traveller a few inches in very light air. Catius suggested having weight forward, which is great, but y'all may need to be farther forward yet. Keep going forward till the transoms quit dragging. You'll know when they stop making any kind of noise. Crew may need to go to the leeward hull as well, forward of the main beam. As for downhaul, pull it down until the sail quits getting fuller; that's your max power setting. Outhaul to flatten the foot a little, I usually want it 2-3" off the boom.
Rembember, Hobie 16's have a lot more sail area for their weight than we do on the 19's, so when it's real light, we DO tend to get sucked out back. Have fun!