Does having an aero downtube actually matter on a bicycle frame? Hmmm...there's been a good deal of debate about that over the last few years, with two pretty distinct views on the subject.
One group, mostly frame manufacturers, says that it doesn't matter because the tubes are drafting behind other tubes or drafting behind the front wheel. There's also the story about a round tube becomes aero shaped if you view it diagonally the way wind would pass over it. Then there's the theory that a real big tube is more aero because the air has time to stick to it and flow around it and off the back of it.
There is this other group that believes aero tubes are better and they (manufacturers) then go off on tangents about how their particular tube shape came about because of a sign from God Almighty or someone even higher up. You, the end consumer, have to read ads and ask opinions but you really haven't had any facts.
Now, as a retailer in the bike business, I have manufacturers call me and tell me all about aero this and that. The big kicker in my case is that I go test these bikes in a wind tunnel to help me make better decisions. Does that mean I only sell the most aero designs on the planet. NO, I still think bikes have to be functional and pretty, so sometimes I sell bikes that have different qualities in ride or handling. Something I don't like though are claims that aren't backed up in any way, so I go testing and try to educate you, the racer.
Should you buy a particular frame just because of the aerodynamics? NO, look for positioning potential and road shock absorption. Ride the bike before you buy it, even if you have to travel a little, then if you're down to a couple of choices, go with the aero one.
Back to that question, do those flattened tubes matter? I went down to the Texas A & M wind tunnel and set up a test to get those answers. I built a test stand that held a 21" long tube at 45 degrees. This is an average length and angle for a 53-57 size frame. I took 4 tubes with me. I got them by cutting them out of frames here at the shop. The first tube was a standard steel 1" diameter downtube, with shifter bosses. Number 2 was an oversized aluminum/carbon tube off of a leading manufacturers frame, 1 1/2" in diameter. Number 3 was an Easton 7000 series aero tube, this is the tube shape that is very popular with custom builders and a few manufacturers. Number 4, I got a piece of aircraft 8 cm wide wing strut tubing. It had an extruded, perfectly shaped foil section and airplane people are supposed to know about airflow, so I thought that would be good for comparison. Number 5 was the same piece of Easton tubing turned around backwards just to test everyone's theory on wing shapes. We cranked up the big prop and watched for the outcome. I've broken the results down to a 40 k time trial distance to help make the numbers more realistic. What this means is if everything is exactly the same, wind, weather, rider output etc. then these would be the differences in the tubing.