One big factor to consider when gauging speed is how old the sails are. I'm assuming you are using a classic T with dacron sails. There is an age of sails where the draft is so far gone that they are more of a drag than a benefit. Yea, they hold air, but they are not very efficient. The main thing you will want to monitor is your VMG to windward. You can go faster, but it may not be productive. In the 90s, I've seen a couple of teams that would benefit upwind by footing. When I was racing P-19s competitively, we removed the cleats off the downhaul and the crew played the downhaul aggressively. A properly played downhaul is golden with dacron sails. With a scrim sail, it will not change much. On my carbon main, I adjust the downhaul about 4" and that is that. When working the downhaul, you must adjust the main sheet as well. I would suggest setting up the boat in light air. Lay underneath the main and pull different levels of downhaul tension. With each level, take a picture from mid-boom to the mast tip. You can print these out and monitor the draft of the sail. It may be that you are not pulling enough downhaul. How much purchase do you have and how much are you pulling?