I lost at least 10-15 FPS running in a maximized window versus fullscreen under 32-bit WinXP. Under Win7 x64 RC1 on the same hardware there is no performance loss. This is the main reason I've been using Win7 as my primary OS lately (alt+tabbing out during the lulls in a raid is essential for staying awake :p).
And yeah, using /reloadui to get external data into WoW is retardedly clunky IMO.
Yeah, the performance loss is minimal. For people on dual screen setups, there wouldn't even be any FPS gain/loss as Windows would still have to render both WoW and Windows.
It isn't really hard or performance hitting for wow to run wow in fullscreen maximised windowed mode without the borders (the option is in game) and just alt-tab to twitter. There isn't any delay in alt-tabbing since its all windowed.