I don't believe Wes ever argued that Valen WASN'T overpowered. In fact, I seem to recall seeing a note attached to Valen saying she was over-powered and to bear that in mind when you download her.
Yes, in some respects Chloe is over-powered. But all said respects are explained quite logically, and thus fit. They also fit in a roleplay sense. And anyone who has played Chloe past Spellhold know that Chloe was very VERY upset about her weapons being tampered with and each player has a valid RP reason to toss those weapons into her general inventory and equip her with some other weapons. You don't HAVE to use those weapons, and Chloe would even give you a valid argument of why she shouldn't use them. It's up to the player's conscience, and thus in extension Chloe's, to decide whether she should take a hit to her pride and use the 'tainted' weapons, for the benefit of the party. So, if you want to use the powered up weapons, do so. If you don't, then don't and you are justified in either decision.
The whole balance debate is flawed, because it seems to expect Chloe to ALWAYS be balanced, but that is simply not possible for her, or any other
NPC. Is Minsc still balanced with the rest of the party if you give him the Sword of Chaos +2 that you get from the Genie in Chateau Irenicus? No, he gains a distinct 'butt-kicking' advantage. The same could be said if you give it to the PC instead. Is your mage still balanced with the rest of the group if you give him or her the staff of the magi and/or the robe of vecna? Sure, you bust your butt for those items, but that doesn't take away from the fact that that mage gains a significant boost in power that the other players don't get.
Chloe starts out a little more powerful than the rest of the party. She's a hardened, experienced mercenary and is not only the child of one of the most powerful deities, but is also favored by that deity as well as another powerful one and has the weapons to prove it. Yes, she can likely kick butt in the beginning, though she still has her weak defense to contend with. I had Minsc and Chloe duke it out about five times, both unbuffed and attacking at the same time. Minsc won 6 out of 10 times, so he seems just a wee bit better than Chloe, or perhaps that extra win could be accounted by luck. It doesn't really matter, what does is that they are fairly evenly matched. Chloe also wins only about half the time against Edwin... as some of you may have discovered under certain circumstances.
By the time you reach Spellhold, though, Chloe is lagging behind in power. Especially if you took a really long time to reach Spellhold, which is best for Chloe even if you're not romantically interested in her. This balances out the early over-powered state, as she is now under-powered. After her weapon upgrade, she is about even with the rest of the party or perhaps slightly better but soon enough she is about even. Playing fairly with no tampering by cheating of any kind, Chloe fits in well with the rest of the party by the end of the game, or perhaps is under-powered even. As several people have noted, Chloe dies fast when getting attacked. Hell, I've seen Korgan with a -11
ac get taken down in a matter of moments by some of the monsters in the game so even heavy plate doesn't always save your behind from getting chomped. Chloe works best as an auxillary fighter, on the side or behind an enemy. Of course, her offensive power often attracts the attention of the enemy she's beating on. Not balanced? Only if you burn a lot of spells to counter her weaknesses. But then, the same could be said of any
npc.