Id say Shisui getting him to awaken Mangekyou is a free power up
No, not really. In the manga, he didn't commit suicide to make him awaken MS. It wasn't planned. It just happened. Neither even had any idea that one could get MS that way, since on the Uchiha Tablets at Nakano Shrine, it says that they have to kill a best friend or someone close to them. Not just see them die.
MS is a free power up to Sasuke since Itachi planned for him to awaken it through his death and this worked. That was one of the main purposes of their fight, along with making Sasuke a hero for taking down an Akatsuki nukenin.
Shisui is the best. He could solo glance you from a mile away.
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No, he can't. He needs to look at your eyes for all his genjutsu, going by the canon manga. Plus, he got fodderized by Danzo. He was good for that generation of Uchiha, but Itachi handily surpassed him in the genjutsu arena by what we've seen.
ot: Sasuke has more raw power, via power ups. Nothing wrong with this, he's a main character.
However, Itachi got that far of his own mentality and he's the best overall genjutsu user in this manga. That title is staying with him until the end of the series. This isn't a big deal, since Sasuke is better with manipulating Amaterasu- it balances out.