I managed to beat all the DLC bosses on my first try except Artorias. Even still, all their combos and super aggressiveness made me feel like I was getting my ass handed to me by Ornstein and Smough again, except in a single boss.
Fuck Kalameet. Apparently I forgot to get him the first play through so I had to fight him NG+ and dedicate myself to getting THAT FUCKING TAIL!! I could have killed him 20 times over but noooooo, he just has to keep his precious tail away from my pointy stick. >.>
He's a complete bastard in NG+7; I died in 2 hits even with armour that had the most defence in the game. I don't know how many times I got him down to 1/4 of his HP and for him to do some ridiculous attack that would kill me instantly...
Armor stats are never really useful in Dark Souls, looks is mainly what matters. (That and weight.) Kalameet on NG+10 and above will most likely one-shot you, yet my builds focuses around never getting hit in the first place. (Either a glass-cannon or a light strength build.) So I've never had that much problem with bosses unless they were amazingly fast. That being said, Kalameet is a annoyance regardless of stats and NGs if you plan on taking his tail.