Ok, Jin yong never revealed how strong Du Gu is, but if the Du Gu 9 Swords is really as effective in breaking qi when practiced by Du Gu would make him one of the most powerful beings ever created. Especially the theory with sword without a sword. Yet, in ROCH was it explicitly mentioned that Yang Guo became as strong as Du Gu Qiu Bai? That is what I was never too sure about.
DGQB would not be as strong as expected (meaning to the level of the sweeper monk etc.) if he is only as powerful as Yang Guo.
I am just thinking, may be Jin Yong himself intended DGQB to be his ultimate character? Coz he's so legendary...then again the Sweeper Monk that guy is something else. Perhaps to illustrate that the ones who are most powerful are humble kinda thing? I mean the most powerful guy is the guy who is a lowly monk who sweeps the floor of the temple.