Aerodynamically, it has the same shape and frontal area if it’s up or down, so no difference.
However, it does put you in a less aggressive position when on the brake levers, so that could hurt your aero. HOWEVER, if that provides you with better climbing and cornering from that position, it may be faster for you.

As a data point, Rohan Dennis runs his BMC basebar flipped up. And he’s one of the best TT’ers in the world.

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Alex Arman

Strava

aerodynamically, probably no impact.

just gets the ends higher. You may find that better for climbing, or not, but if the bike was specifically set up to match your fit then I'd say leave it as is unless you have a compelling reason to change it (e.g the fit is bad)

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I cant imagine a noticeable difference in aerodynamics for the (the same) bar in either installed state. It would be a symmetrical airfoil shape (unlike a plane wing which is asymmetrical to create lift); there may be some difference with low stack configurations compared to high stack but I have no data to support that.

IMO, The bigger difference will come when you are riding with hands on the base bar (e.g., climbing, technical descending): the flipped up bar would put your torso in a more upright position which would cause more drag. I personally think it is more important to have the base bar in the position that gives you the most confidence during descents, and allows comfort during climbing.