It is possible that Kano improved upon Jiujutsu, but since it would essentially be a derivative, I find it hard to believe that it would be more combat effective.
Diato Ryu is a form of jiu jutsu, BJJ is a form of Jiu justsu...I do not put them in the same category as aikido really if you look at it that way.
The reason why many (myself included) feel that judo training is better than traditional jujitsu training is because of the emphasis on randori. Kano took all of the stuff that couldn't be done full force on a partner and put those into kata, and used what was left for randori.
BJJ came from judo, not jujitsu.