As was pointed out to me by Claude B in the comments section of a previous post, Argon has a new aero frame out, the Nitrogen (web page here), and with it they show some wind tunnel data:
These tests are following a pattern. The Cervelo S5 remains the standard against which new frames compare, and they may be competitive with the S5, but don't beat it. We saw the same thing with the Parlee ESX and Felt AR1. Then lagging behind is a set of usual suspects, including the Specialized Venge and the Scott Foil. The only test I saw where the Scott Foil did exceptionally well was the test data produced by Scott itself.
Between the Nitrogen and the S5 is basically a choice between low-yaw and high-yaw drag. The Nitrogen appears to do a bit better at higher yaw. I tend to err towards lower-yaw performance because of ground shear: yaw at the height of a downtube is less than it is, for example, at head-level, let alone at 10 meters. At extreme yaws control becomes a bigger issue than simple drag.
A big factor on the aero road frames has been comfort. The Parlee ESX and the Felt AR1 have both focused on the seatpost for that: the Parlee with a cool bent post and Felt using a novel seat clamp to allow for a very thin, relatively flexible aero post. I'm not sure how the Argon stacks up here. But the most important factor in comfort is tire options, something Cervelo realized in the latest S5: clearance for at least 26 mm tires is important.
One feature of the Argon which reminds me of the Parlee ESX I test rode is the integrated headset spacer. Maybe it's more aero and maybe it's stiffer, but it certainly looks better, than simple round spacers.