Deep downwind is not the fastest point of sail, but about 120-130 degrees off the wind is. As you transition from a beam reach (90 degrees off the wind), speed will increase until you max out in the 120-130 degree range, then your speed will begin to drop as you run deeper and deeper. It's the loss of the apparent wind as you point closer and closer to the direction of the wind that causes the slow down.
Regardless of sail size you will always be faster on a beam reach compared to a deep run. Formula racers carry the giant sails to maximize their downwind runs. They may use a 9.5 meter sail in 25+ knot winds for downwind speed, but they would likely get hammered if they tried to do a beam reach in the same winds with the same sail.
You are right about releasing the outhaul, it stabilizes the sail and increases the foil as the apparent wind drops. Sheeting out? Some for sure, but not a bunch. Stay in the outside straps until you think you will be killed, then go for the chicken strap (outside straps are ton's faster). Make sure your boom is high, otherwise you will feel scrunched up when you are in the straps with the sail racked back as you head downwind. Harness lines long for downwind (short for upwind).
Hopefully, some others will jump and add to my comments. I am a good formula sailor, but not the best around by any means.
It takes a lot of practice to get comfortable on the downwind runs with big sails, especially if there is a lot of big chop. Damn scary at times.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.