first before sasuke killed orochimaru he said that someone who was considered a genius to the rest of the ninja world was nothing when compared to an uchiha. second when orochimaru tried to get itachi he made the mistake of using genjutsu, an area we know that itachi excels in. he wasn't expecting itachi to be so good with genjutsu and he got pwned and got his arm chopped off in return.
true one of akatsuki's goal was collecting jutsus so itachi must have a nice arsenal but don't forget that orochimaru was the man who wanted to discover the truths universe and learn every jutsu. so orochimaru was also doing what akatsuki was doing as well. well what do we know about orochimaru, oh yea that's right he trained sasuke so it's safe to say that sasuke definitely has a pretty good arsenal of jutsus as well.
what do you mean clones are much weaker than the originals. that was itachi's kage bunshin. kage bunshins are supposed to have an equal amount of the original's power. if kage bunshins are so weak then why couldn't naruto easily defeat itachi's kage bunshin that he encountered. also don't forget that it took nagato, yahiko, and konan three years before all three of them could kill jiraiya's clone. also at that time jiraiya prolly wasn't considered a legendary sannin and he was also young so there was definitely time for him to get stronger.