I personally think that since they were apparantly so close that Itachi saw Shisui as an older brother, that maybe just seeing him die or knowing he's not coming back will be enough to get him the power.
Just like when Itachi and Sasuke where fighting, Itachi was already dying and even then he was still stronger than Sasuke. Itachi was walking - half dead - to Sasuke (who was backing up in fear) when it seemed as if Itachi was going to finish Sasuke off, he instead transfered some of his power to him then gave into his pre-existing illness and died. So basically Sasuke didnt kill him yet he was able to gain the MS after finding out the truth, and that his brother was only trying to protect him.
So maybe a new way to get the MS is coming into play. Maybe instead of killing your best friend, you loose someone real close to you, more likely if you see them die, your emotions about that could cause them to have a certain power surge.
<source> M overactive/running mind.
