RiverKid has a 21' Shiada. The set back is about 12", not sure what prop it runs. It runs about the same speed as yours.
I almost bought your boat. You got it from the Saunders? If you are only getting 5200 Rs out of it...... I would suggest going to a smaller prop maybe 21 or 22. I don't exactly what rpms they are supposed to run, but I would figure (and someone correct meif I'm wrong) around 6000. That and moving the motor up a bit will help. Remember Shiada's are solid boats and it takes a lot to get them going. As far as a set back, the futher from the transom, the higher the motor will need to be. I think his boat has hydraulic steering. Not sure how much you want to spend on the boat (hydraulic steering is about $1000). I'm going to put it on my boat over the winter, I hate the torque steer.
How does the boat handle with the current set up? Does it get up on plane good? This will help determine if the motor height is correct.
Here's the boat I just bought:
http://www.hillbankusa.com/signfx/gmt/DSC00008.jpg
http://www.hillbankusa.com/signfx/gmt/DSC00012.jpg
It's a 19' GMT w/ 2.4 Merc Offshore 240hp. It spins a 24 to about 5900 to 63+mph. I am going to try a 22 and see if I can get 6500 and a bit more top end out of it. The boat is layed up like a tank.