She didnt have to go full power, she went full power because it was the quickest way to deal,
Second, we don't know how powerful irene made them so to claim that they are/aren't isnt right,
Third, when she did go full power they were terrified by her magic power, like shitting their pants scared, they called her "a monster", knew they had no chance at all. People always disregard that, meaning her power (in algeria) completely overweighed theirs, so im confident that another one of her forms were capable of beating them, so we don't know the true power of Mirajane once again.
We dont even know what Algeria can actually do (we'll probably never see anything just like Stitri), Hiro just likes to troll us and only show off her Base form powers for some reason and then tease us with a powerful form and only shows us a punch, lol.
I Think the safest thing is to just assume she's on base spriggan level -- Ajeel/Jacob (and others) until we get to see more power from her (Sayla and Algeria).
But based off what we have seen, the only characters i would place above her (not including spriggans) are the main 3 - Erza, Natsu and Gray, and the obvious two Laxus and Gildarts.