Ok, it seems to really show itself when the layout size is changed, but not the application size. Here are some samples:
Default 1024x1024 application and layout:
Now only the layout is set to 1024x768:
And now both application and layout are 1024x768:
Some visible difference right there. And the fps shown is pretty much an average, I waited a moment for it to stabilize.
Edit: Also just noticed that it suddenly starts to need 4 Megs of VRAM in the second sample - why is that? Sort-of drawing the application "background" and the layout as different things or something like that?