Running KDE on a laptop and also possess a larger 34 inch monitor that sits on my desk. Have my system setup so that when I connect my laptop to my monitor the built-in one turns off; in other words, I use either the laptop's display or the monitor on my desk, but not both at once. I like the look of Gnome2/MacOS and think it works well on a small laptop screen, but having multiple panels on anything larger feels like wasting real estate and thus prefer KDE/Windows' default layout. Unfortunately, this does not seem possible to do and KDE applies the layout that you set to the primary monitor. If I try to change it, then switch monitors, the change is propogated to the other monitor.
That's how it works at the moment, yeah. In the past what you wanted was possible, unfortunately it had many side effects that were difficult to understand and also difficult to recover from: issues like desktop icons places on the screen disappearing when using the other screen; losing your panel; needing to manually sync widgets and panels from one screen to another due to lack of auto-sync, and so on. In the end we had to remove that feature because it just didn't work well and it caused too many support issues. So I'm afraid we won't be bringing it back, sorry!