You have never seen the frame rate dip substantially? Forge World makes up 80% of the maps in matchmaking so I'm not sure how that's possible. Tempest, Anchor 9, every Firefight map, every Forge map, Zealot and Countdown, every BTB map, Condemned (low-grav room LOL) and basically every map in the game will break down. This is after Bungie promised a completely re-built engine to ~*push the boundaries*~ of the Xbox 360.
The difference between you and I is that you're willing to accept any frame drops as long as they're not "too bad", I want a rock solid, flawless framerate. I see stupid shit like mechs being built inside a level and just know when nades start raining and rockets are flying around that I'll be trying to snipe and it'll be crawling along at a cinematic frame rate. Hope I'm wrong.
Never in competitive (co-op is another story, so I hope that they switch over to the same model they use for competitive in Halo 4.)
What do you consider substantial? A lost frame or two isn't that noticeable, even at a 30fps running speed. I've *never* had a game where things lagged to the point that it affected actual gameplay, even in the low-grav room. The fact that I've only ever heard you complain this loudly about it makes me think this is atypical. Halo 3 will lag much easier and much more.
Not saying a framerate that stays solid isn't desirable. But I don't think things are half as bad as you make them out to be with every post.