So first off, I don't recall any companion claiming the tadpoles de-powered them from what they used to be before. If there are conversations to that effect, I'd be interested in knowing more as I don't recall any such thing.
So this is my take on the Lvl 1 issue:
Lae'zel: she comes off to me as book-smart so to speak from her training but woefully unworldly and kind of naive. She ascribes a lot of noble purpose to the Gith and her Queen that just isn't realistic. (She has seriously drunk the kool-aid.) We also have no explanation as yet as to where she was when she was abducted or otherwise how she was on the ship. She will point out her creche up in the sky so I think it plausible she was abducted from there. This would explain her lack of experience and practical knowledge. She was the star pupil who memorized the textbook but has yet to really put any of the theory she learned to the test. She was abducted before her training was completed.
Gale: As others have mentioned, he had his tryst it seems with Mystra, got spurned, and now has a bomb in his chest. Either, (or both!), could account for his lack of juice. I can buy his claim of having been more powerful once. We may yet find out it is less true than we think but I'll go with it for now.
Wyll: I view him as we might say here in Texas, 'All hat and no cattle'. I think his reputation far outstrips his actual deeds. Maybe a couple of his exploits have a kernel of truth to them or were facilitated by Mizora to entice him with the power he might one day wield as her servant but I think most of it is hot air. That said, I do think the one true thing of him is his desire to be seen as a hero and live up to his inflated reputation. He should arguably be a Pact of the Blade too in keeping with his persona.
Astarion: He may have been around for centuries but he's been a slave on a short leash. His 'experience' is all as a seducer and ratcatcher. So you could argue his skills should in some way reflect those aspects, but an adventurer with real levels he is not.
Shadowheart: She's had her brain erased and is potentially a mindwiped cleric of Selune. If so, it is no wonder she is low level. She's had her brain formatted and her divine connection hijacked. My theory is that she is indeed a captured Selunite and the tadpole and it's magic is slowly eroding what the Sharrans did to her. In time, she will start to remember and have an internal conflict between her real self and her programmed self. Which wins out will be part of the RP with her as the game unfolds. She'll either regain her old self and memories or reject them as some aberrant effect of the tadpole.
