Inhaling Sulphur Hexafluoride.

If you mixed some oxygen (20%) with this stuff then filled a large sealed room: would you be able to fly with a well designed pair of wings?

Nice try (he he). SF6 is only about 6 times denser than air and 100 times less dense than the average engineer. Better leave about 0.1% CO2 in there as well?

I was hoping to fly not float: 40 lb kori bustards (and some swans) are about 500 times as dense as air, and, manage to fly quite well. Why do you say I should leave some CO₂ in the mix?

CO2: the idea was to maintain acidity in the blood stream. It's probably not beneficial while flying, as you will create enough CO2 yourself, but would be helpful during preparation and recovery, so you can raise the O2 level in the blood stream without causing repiratory alkalosis. The alternative is judicious use of a paper bag (but you might need some kind of CO2 measurement to supplement natural responses in this unfamiliar environment).

Even with a dense atmosphere, I fear the basic problem of muscle strength and energy provision (oxygen) versus body weight may not fully be overcome. (The problem is during take off from a flat surface). But you are right that the relationship between lift and wing size (and also viscous loss) should be improved with SF6.