Its not really about the plane flying straight, which does help ofcourse, its the pilot who is at the sticks. you may be surprised when you get a "straight" plane but it just doesn't give you the boost you were expecting.
i don't know the whole story, and may misjudge slightly. but your performance at a competition determines what you need to practice, be cause it puts you under stress and the weak links always break under these
circumstances.
If you ask me the best way to determine where you are going wrong with your flying it so get comments from judges and fellow competitors and that is really regardless of whether your practice with a caller or w/o one.
some one may say your plane is actually not that bad its the setup... or your wings are never level and thats why the upline is always off... or you are giving aileron input together with elevator and thats why your loops
are corkscrewed.

just my 2 cents