I have the smaller one. I have a sprint tomorrow with an 11 mile bike leg. Is it an aero advantage just to have it mounted? I wouldn’t need to put any drink in it for just a short burn. I was thinking just have it mounted and capped off.
I thought it was pretty non-aero…but I’m not sure about the particulars. It could just have been that the straw was the problem? If you have an aero head tube like some of the Uber bikes out there then it’s probably bad. Perhaps on a standard round head tube it helps? But that would probably only be at zero yaw which is not a realistic situation in an outdoor out-and-back course.
You’re pretty strong though. You can stick it to your foes by beating them without aero gear. 
If your bike has an aero shaped head tube leave it off. If it’s a standard round tube put it on. Either way you don’t need water for an 11 mile bike leg.
With a standard headtube it’s neutral (at best), and with a shaped headtube it’s worse.
If you are not putting water in in ditch the straw, and make very sure it’s close to the headtube, and not angled off to the side.
I remember reading something from John Cobb’s recent tests… “Aero Bottle, not so aero… But don’t forget the bento box.” Something like that.
Basically, it’s tough to split the wind any better than the bike already does, but you can clean it up as it leaves.