Well, I don't have all of those applications installed, but I'll do a little test for you- since I'm not sure whether or not it'll be possible to upgrade directly to the GM, I haven't installed many apps, but here goes:
I currently have these applications opened:
-4 Finder windows
-App Store opened, downloading two applications
-Four pages of Safari, each with a few tabs.
_a news page with CNN, WSJ, BBC and CBC loaded, along with several stories from said news sites opened in the tabs
_a work page with two Google Finance pages and my work's intranet site loaded
_a tech page with The Verge, Gizmodo, Jalopnik, Engadget and AnandTech open
_a 'random' page with a few MacRumors threads opened as well as two high res image heavy sites opened (wallpaper websites), and of course the usual Facebook/Twitter tabs
-Mail opened, refreshing three email accounts
-iPhoto with my photo library loaded up, ~1000 images
-iMovie with a few short clips loaded
-iTunes, ~5000 songs, music playing from a Genius playlist
-QuickTime open with a 1080p video loaded and playing
-VLC nightly build open playing a different 1080p video
-Disk Utility restoring Lion InstallESD.dmg to a connected flash drive
-Activity Monitor loaded- showing 55% CPU usage and 13GB RAM in use
...and yes, there is a bit of lag when opening Mission Control. It's not awful by any means- it's not buttery smooth, but it isn't so slow that it locks up the machine or something like that. I can't say when I'd ever open up two 1080p videos while editing a video in iMovie, but if you do that, you might experience some lag in ML
With all the extraneous stuff closed, with only the Finder windows/AppStore/Safari pages/Mail/iPhoto/iMovie/iTunes opened, it becomes buttery smooth again.
At no time was there any lag in Safari, even with all the pages opened (I'd be pretty upset if those pages lagged Safari esp. since I basically run the same pages/tabs on my slower Alienware laptop daily with no issues in Chrome). Might be your number of graphical heavy sites that's causing lag?
Both 1080p videos played smoothly and using the UI was still smooth. The only lag was when opening Mission Control or opening the Expose view of all opened windows within a specific application.
In practice (for my usage) it's just fine, but I can understand why you might be confused as to why doing the same thing lags ML while running smoothly in Lion.