I've got a mini 1.42 hooked up to a 2005FPW. Tried portait mode once when i just installed it. Quite easy to change the setting and turn it, so easy i'm now writing this post in portrait mode, specially for you!
When moving around windows i do see 'refresh lines' running over the screen, normally they would be going top to bottom, and i guess they aren't that obvious then, right now they are very noticeable.
Windowserver hits up to 30 % percent CPU when moving windows in 'normal', when it's at 90° it double's that. Switched back and i have a really hard time trying to see the 'refresh lines' in normal mode.
Strange how this puts more load on the CPU. I like my screen in landscape. Thankfully.