This plane flys very well at the recommended CG. Mine is about 3/8" aft and it's still a *****cat. Moving the tank will not change the location of the proper CG for the airframe. It may require some minor adjustments, but the proper CG location for any given airframe is, for the most part, a constant. (I think you are asking if the tank modification alters the 'optimum CG point...it does not0.

Have fun. This is a very good airframe.

(I really can't believe that word was 'asterisked'-out) How *****, *******,****** and ******* paranoid can you be? (just kidding)