FPS definitely suffered, it just wasn't crippling.Blighttown has a known FPS bug. Some PS3 users experience it. some don't. Does not happen on 360/PC afaik..........
Yeah Sens Fortress was an awesome area. One of my favorites of all time in a souls game.
Ornstein and Smough are, by far, the hardest bosses in the game. I've only beaten them once solo and it involved a lot of strafing around the room and hitting them with Sorcery. I killed Ornstein first since Smough is much slower and his attacks are easier to avoid at range. You need to keep the pillars between you and the target the whole fight to block attacks as much as possible.