We are having a discussion on our boat about mainsail trim when running downwind. I generally let the cunningham off and loosen the tension on the foot of the main. The debate is that a loose foot reduces the sail area and therefore potentially the speed??
Loosening the cunning ham is a must in most of the conditions unless it is too overpowering. You need to be very careful while playing around the foot. Generally you must consider the sea state (flat, choppy, swells etc) when you play with the foot. In general, in swelly conditions you should keep the foot loose and in flat water you can keep it tight. Hope this helps