Yes, I'm sure it's possible to hurt the speed of the Stealth by sitting in the wrong place, and forcing the rudders to do unnecessary work (just like any other boat).
It's easy however to find the right place to sit... if you pull the securing pin out of one of the rudders, when the T-foil is running parallel to the surface and doing no work it will just slide up and down in the stock as you go over waves. If it's permanently stuck down you are too far forwards and if it just slides up you are too far backwards.
However because the T-foils compensate, fore and aft trim though still important, isn't as critical as it is without the T-foils.