FWIW I don't think a single speed on a flat course is the worst thing in the world. I do a weekly group ride and there are a couple of guys riding fixies that I've got to really bust my butt to keep up with.

On a hilly course it puts you at a disadvantage because even if you can hammer up hills pretty well, you don't have the big gears to fly down the other side.

Anyway, as far as aeroness goes I wouldn't mess with your setup at this point. You're right, you want the bars lower than the seat but you then you may have to slide the seat forward so you don't loose power. You probably don't want to tilt the seat down. That's going to put weight on parts of your body you don't want weight on.

There are other little aero tricks that I can't remember now. MIT has done some studies. Even the way you put the race numbers on your bike can make a difference.