Welcome to the Hobie Site. With regard to your questions, I can give you a little info.
1. Yak speed is based mainly on the length of the hull at the water line, so the Mirage drive will not necessarily give you a greater speed. But it will most likely give you a more sustained and constant speed over longer distances since the legs are generally more powerful than the arms.
2. The pedal drive fins or sails do indeed fold up flat against the hull when entering skinny water. Or you can pull the drive entirely while on the water. Takes all of 10 seconds. See first pic below.
3. With the new larger hatch, you have pretty much complete access inside the hull from the bow. Some folks mount their sonar battery next to the mast tube, and there is plent of gunwale room for the actual mounting of the unit itself. See first pic below.
4. Sail works great on the OB--plenty large enough for it to get up and move. See second pic below. Don't know why you would want to mount a sail on windless days though, but you could certainly do it if you were an eternal optimist!
Outback a great boat for just about anything, but if you are interested in more speed, you should check out the new 16 ft. long Adventure as well.