We regularly sail with 7-8 of top level racers together (some PWA/Olympic), lining up and comparing speeds, some of the key points for me:
Downwind foot in or out the straps makes only a slight difference in speed. In the straps gives more speed but higher angle, out the backstrap gives a little less speed but lower angle. Things change when you're pumping downwind, in the straps you can pump harder. I feel in the straps has a higher VMG when powered up, if you can handle the power (for me medium wind, 10.0 in about 10-12knots, 9.0 in 14-16 knots), but lower VMG if you are under or overpowered, due to having to sail a higher angle underpowered and being out of control overpowered.
Staying hooked in is a must, it increases power, but also allows you to hang your weight in the sail / pushing down on the mastfoot. INCREASING STABILITY! To keep the lines at the correct place for up/downwind you NEED an outhaul kit. Trim your lines to be in the correct position for upwind (you can practically always give full outhaul upwind, independent of windstrength). If you did it right the power should be on the front hand when going downwind, meaning you can trim the balancepoint by letting off outhaul. (Releasing outhaul shifts power in the sail back - creating balance over both hands downwind)
Same as upwind, bending the knees or bending at the hips decreases power enormously. Fastest is to stay completely straight and extend yourself from the rig as far as you can, as long as you can without being overpowered. In theory it should be doable in any wind by correctly doing the next point..
Sheet in going downwind, independent of windstrength, go more downwind if you have too much power, go up if you have too little. The goal is to find the line where you are powered up (able to keep the board flat), but not overpowered.
Lastly, use the gusts downwind to power up / launch yourself into "the next gear".
As for trim, things dont change much..
Trim range for various settings:
Mastfoot 3cm further forward in high wind
Downhaul 4mm tighter in high wind
Outhaul thighter 1-2cm upwind, downwind no idea, dependent on where my lines are!
Harnesslines move back when wind increases by max 1cm
Harnesslines a little longer when its windy, max 2 inch.
Mast rake .5degree more when its wavey, not per se when its more windy
Stab angle = same independent of windstrength, max power
Straps = same, although I'm at maximum width for the straps, and step further back(half out of the front strap) in light wind, and sometimes out of the strap on the rail in front of the strap with the backfoot in light winds to be able to keep myself straight without sailpreassure. (I need sailpreassure to be able to sail with both legs completely straight and both feet in the straps) so I dont really always use my straps when its light? Kinda like: