i can not confirm that this does work. as i tried that too, i set the aspect to (16/10), started the engine with fullscreen 1280x800, took a screenshot and everything is streched on the screenshot. try setting the aspect to 1.2 in 1280x800 and take a screenshot and be surprised...
try it for yourself if you dont believe me.

as far as i understand it doesnt matter what you see on the screen, you have to produce correct output on the gfx card. you could see a totally distorted picture on your monitor, while the engine still makes nothing wrong. just check that with a screenshot.