What I've done to make the windows that are too big accessible, is to go into Control Center -> Regional & Accessibility -> Keyboard shortcuts -> here you can assign buttons to do various tasks while in KDE. For example on my N810 there is a "menu" button to bring down the menu in Maemo apps - that is pretty much worthless as is in KDE (KDE just recognizes it as F4) so I assigned it to "Move Window". Now whenever I want to easily see the bottom of window that are too long (mainly so I can tap OK/Apply/Cancel etc...) I just press that button and then use the stylus to move the window to wherever I want it. I've also mapped the "Home" button to "Minimize" and "Shift + F4 (menu button)" to "Close" and so on until all of my buttons were mapped to what I want. You can also do alternative assignments for each of the commands which is handy if you're using a BT keyboard so using the keyboard uses buttons/keys that are convenient to it and my BT keyboard has it's own set of shortcuts.

The ability to make keyboard shortcuts is definately an advantage of having an N810 - not sure how simultaneous key presses will work when using the virtual keyboard.

As for speed and usability - I personally use KDE exclusively. Perhaps my desktop just plain stinks but I don't find KDE that much slower than it at all. I haven't used the Maemo desktop in about a week. I've made desktop shortcuts for all my favorite Maemo apps (Canola2, FB reader, SDict etc...) and I find use KPackage to be better than the Maemo Application Manager. I've got the shortcuts to Wifi connection and Maemo Control Panel in my system tray - so there's really no reason to leave KDE anymore.