I don't know why there are so many references to the "real world" anyway. Birds can fly because of reasons none of these other races have except for sylphs... In the real world, they'd all be too heavy to lift themselves even if they had gigantic wings. I think it's safe to say, if they have wings, they can fly equally well, since technically, none of them should be able to fly anyway. Why not give the races a wing size variable, and then use it in relation to their bodies. If they have big wings and small bodies, they can fly higher than things with big bodies and small wings. I think that's the only logical way of looking at it. I also feel as Harold said, encumbrance should factor in too. Someone carrying 500 stone, can't fly as high as someone carrying 0 stone.

If we look at the carry system, everyone starts at 0. You don't have any problems lifting or carrying yourself, so I don't see why that shouldn't apply to flying.

I think a wing size variable, compared to body size could be a base determinant of how high a specific race can fly, then use encumbrance to negate some height.