^Sitri isn't an evolved version of anything. It's its own Demon.
Satan Soul, like Beast Soul and Animal Soul, is just a category of Take Over. There are multiple Take Overs within that category, no different from Lisanna's various animals or the new Beast Souls Elfman showed just last chapter. [Hence why Erza mentioned it as being the strongest Satan Soul out there]
So yeah...Mirajane used an entirely new Take Over within the category she's chosen. Just a more powerful demon than her original Satan Soul. [Wouldn't expect anything more than a variety of demon/Satan Souls for future reveals just as I wouldn't expect any of the other Strauss's to do anything outside of their own Take Over specialties]
I have no complaints on the fanservice, really, but I also wish the fight was a bit more explosive. But who knows? That might have just been too much for a mere Tourney fight. Maybe when she's involved in full-on serious combat again.
But still...we also got a bit more on the Fiore Govt. as a whole. [Like FMA, many seem to be in on the conspiracy...though it's unknown if the King is aware]
But yeah, the matchup being alluded to for next chapter should be interesting. I'm guessing one of those two names is the female Sabretooth member we have yet to be formally introduced to. [Though the feathered outfit and white hair kinda makes her look like Angel, though I know that is incredibly unlikely. XD]