Yes. The adjustable upper castings have the screw and it is so you can adjust rudder rake without having to re-drill your rudders. As far as answering your other questions about proper rake, I'll have to leave them to someone with more technical knowledge that I have. For instance, many racers carry a certain measurement of mast rake and there is a way to get that measurement.
But then again, it may not be your rudders or your mast rake. Sailing too close to the wind (or pinching) will slow you down, and create a lot of lift. Over-sheeting can also cause you to slow down on certain points of sail.
If possible, I would try to find someone local that has been sailing a 16 for a while and let them give your boat a good look over. Maybe even sail with them and see how they set up their boat.
In addition, this is a wonderful site for catamaran sailors, but if you also post your questions on the hobie 16 forums you will likely get many more responses.

http://www.hobiecat.com/community/index.php?c=N