I have no credentials either in sailboat racing or in fluid dynamics, but I have observed a bit of what both jzk and quirkster are talking about and I have been trying to assimilate it.
1. I had noticed that easing the mainsheet until the sail begins to luff, then pulling down on the clew seems to improve performance on both a close and a broad reach in lighter wind, but it has not seemed to make as much difference in stronger winds.
2. In stronger winds, when the boat starts to develop weather helm, I have found that easing the mainsheet usually brings the boat into balance, although the sail trim is not "perfect" in terms of the windward telltale streaming. As the wind gets stronger, I have found there comes a point where, to counteract weather helm, the sail needs to be eased to the point where it is really luffing (ie flapping around). I have taken this as the signal that I need to reef the sail somewhat. This also seems to be about the point where the daggerboard actually starts to make a difference. I like this technique of using the sail trim to counteract weather helm in gusty conditions as it is a pain reefing and unreefing the sail as the windspeed varies.
These are just my observations and I make no pretence to expertise in these matters. I would like to ask quirkster how much he feels the sail should be allowed to luff when sailing close to the wind. I confess to having an instinctive liking for a neatly trimmed sail - at least not flapping around, but I'm open to advice.