I just did the exact same thing with an Edge rod I bought. Its a straight 12" carbon fiber handle with a normal butt cap and it felt too short and like there was nothing to hold onto when I swung it back to cast.

Called Batson and had them ship me a piece of scrap blank to put in the butt to extend it along with the cert. pro fighting butt style you mentioned. Removed the butt cap, epoxied the scrap blank in the bank to add another inch and a half and then glued on the butt cap. It gave me better grip, added length to the handle, and helped balance out the 11'3" rod a bit better with the counter weight. Made a near perfect rod into a perfect rod! I think you're right on with that idea.