Can you get higher than a Sextuple Kill?

Let me describe a very specific situation to you,

Let's say you're in a game of Overwatch and you kill someone. Right after you get that kill a Mercy starts reviving them. While she is reviving them you are getting kills to perfectly keep your kill streak going. You get to a Quintuple Kill and then turn to the Mercy, you kill her to get a Sextuple Kill, but the thing is that she finished the revive on her teammate. You kill him just barley keeping the streak going.

What would happen? Would the announcer say that you got a Septuple Kill? Or would she just stay silent?

Yes and no. The precise scenario you describe would technically be a septuple kill. However, there is no voice line for a septuple. The announcer called the sextuple and stopped even though there was clearly a 7th kill in the set. Since this scenario is extremely unlikely, especially after the Mercy nerf, it doesn't seem likely that this will be added either.