Isn't it time Win8 is added to the list of OS's? Who knows, someone might actually use Terragen with it....
Now seriously: I did run TG2 on the Dev Preview and have to admit it did run very smoothly. In my enthusiasm to test Win8 I forgot there was a render on its way and it wasn't till after I installed a couple of programs that I did remember the render... Didn't notice a significant slow down while running the installers. On Win7 I would never try to run anything next to TG2 because I did try once and the installer took forever to finish.
And about Win8: Well, that's not running on my machine anymore because I think it's fine for smartphones and tablet-pc's but not for the traditional desktop/notebook.