Other than massive lag when the dragons are spawned on top of you, performance seems fine, it drops to roughly normal performance with 50 of them flying in the air.
Specs: I have a single GTX580 and a slightly OC'd i7 2600k, playing with all settings on Ultra (and a couple of INI tweaks to add extra shadows on trees/terrain) at 1920x1080, and was for the most part at 45-50fps. Turning on FRAPS to record the video dropped FPS to 33-35 though. For comparison, in a normal scene I'm generally at 50-60fps.
I'm also running the mod to allow borderless-windowed mode, which might be screwing with FRAPS a bit.
And both FRAPS and Skyrim crashed when I attacked a dragon :/
So while I wouldn't say the game is stable doing this (at least with fraps + borderless), the engine's performance still seems great on a machine that isn't exactly a budget build, but isn't anything too exotic either.