The only way to survive DW on the 20 in the big stuff is go way deep, DO NOT try to heat it up, 20's will pitchpole (for that matter any cat will pitchpole in big wind).
Just wanted to chime in with some tips in sailing a 20 in heavy air. Before acquiring my Tiger, I raced a 20 for many years. Key upwind settings - let the main traveler out about 6-8 inches from center and sheet in HARD. Set the mast rotator so that it points at the rear crossbar/hull corner. Keep the slot open between the mast and jib but don't let the jib luff. Have the crew continuously work the downhaul when the hull raises. Get on the wire (both of you) and sail a trimmed course where the windward hull is just kissing the water...keeping the boat FLAT.
When transitioning to downwind after rounding A, try and keep your speed up in relation to the wind as you bare away. This will lessen the impact of the wind hitting the sail with full force. Do not travel out very far...use the upwind setting. Keep the main sheeted in tight and sail deep. This allows for minimum sailplan to the wind (more of a "glancing blow"). Play out the jib so the tell tales are flowing. If you can...get your boards up (I have my crew pull the windward board just before rounding the weather mark and she'll pull the other one after we round). The boat will trip over itself if the boards are fully extended at speed. For security...get one or both of your feet under the tramp strap. Sailing deep downwind in this manner requires concentration and minimum tiller movement
When gybing, do not make an abrupt turn to the new course. Start to head the boat up ever so slightly until the main gybes over (you might help it by grabbing and forcing over with you hand) and continue to sail very deep on the new course. This is where a lot of boats lose it and end up capsizing. When entering and exiting the gate or C mark...this is where you play out the traveler, main and jib...keeping the boat flat as you spill wind and turn windward at the same time. As the windward transition is being completed, return to your upwind settings.
Usually racing in heavy air is a race of attrition. Your not very fast when you upside down. Concentrate, keep the boat flat, be deliberate in your manuvers and you'll finish well with the pointy end up.