Well the thing is ... they are not so super-powered actualy.
I mean yes, Astarion is old ... REALLY old ... But thats it.

That is his whole expertise ...
All he ever did was to appear on some culture action, charm some foolish noble lady and then lead her to his masters lair ... and i dont made this up, that is what he said to us.
This kind of life, no matter how long it is, dont give you much "adventuring experience". :P
Wyll ... i mean, yes he told us that he fighted hordes of incredibly strong enemies ... but once again, thats all, only HE told us ...
I know it might seem little odd that for one companion im quoting his own words as proof ... and for other i use them as disclaimer ... but concider others ... Goblin by Windmill, Spike, that Flaming Fist captain ... they all know him personaly, and they all concider him useless poser ... maybe those stories about his heroic deeds should not be taken too seriously.

Also, note what Volo did out of that attack on Grove gate.

Gale ... well, this story should be self explanatory.
Since his orb consumes magic, i believe it dont sounds so unbelievable that he simply lost most of his power bcs of that.

Some might question why is he even able to cast any spell ... and i dunno, i dont created that character, but if i had to guess ... i would say that as we level, he slowly learns to get control over how much he is able to restrain it.
The only companions that is supposed to be stronger than they actualy are ... are funny enough Shadowheart and Lae'zel ...
Lae'zel spend her whole life training and teaching to be as effective soldier as possible ... that is whole life purpose of her kind ... yet her power is exactly the same as any other fighter we create and technicaly lower than most fighters we meet.

And Shadowheart just finished (not sucessfully tho) secred mission to obtain the artefact, so she should not be so lame logicaly.
