We are talking about different things. I was referring to the idea of using a faux FS method. The performance hit I am referring to is this.
Anytime you upscale a graphic it eats performance because it has to scale and redraw, probably at least bicubic so it doesn't look like crap. FWIG is that your resolution would not change but use a buffer that pretends to be a fullscale window at say 1600x900 then upscales it to your native 1920x1080 (at fullscreen) leaving your desktop alone. My AMD card sorta does this at least to the monitor, it scales in GPU so the monitor doesn't really change resolutions, not that useful actually.
"Bad performance wise"? WTF are you smo.... oh for games, yeah that.