Yeah, downhaul is a must to be 'fast'. To be 'fast', when there is plenty of breeze, you need to control your throttle every few seconds and the main downhaul is the fine control on the throttle. You want to avoid steering up into the wind to depower as this requires a significant movement of the rudders (drag) and dramatically decreases the efficiency of the sails when you point up too high.

Downhaul works on any boat but it really came on when our boats designers started raking the diamond wire spreaders aft giving the mast "pre-bend". With this system, when you crank the downhaul, it bends the mast significantly more - which flattens the sail by moving the mid-luff forward. However, even on non-prebent systems, stretching the sail material with downhaul also serves to move the draft further forward making the sail more efficient and a little less powerful.

So yes, I suspect you will experience quite a difference when you are trapezed. When a gust hits, quickly tighten the downhaul and you can literally feel the boat squirt forward as the sails become more efficient and turn what would normally be excess power into speed. Without tweaking the "throttle" this way, your boat just heals excessively and you have to pinch up to keep it from flipping over.

6:1 and maybe 4:1 should be plenty for a dacron sail but you probably need to start thinking about 8:1 or better for laminate sails.