The AR models are made with the same BB stiffness as our F-series models which share the same stiffness for production bikes as the frames we supplied to Garmin and now to Argos. You can find a few resources online from Roadbike, Tour, ProCycling and others that rate the F-series BB stiffness among the highest available period. From a stiffness to weight perspective, only the Cannondale Black Supersix EVO and Cervelo R5CA (and now RCA is claimed) are as good as the F-series.
If you don't want Di2, the AR3 R or AR1 frameset would be a good choice. If you plan on TT events as a dual use bike I'd recommend the 51cm size. A 54cm would give you a huge head tube to deal with on a TT bike.
Thanks for the reply,
Instead of the AR2, the AR4 is the bike that would give me the best options with mech Ultegra. If I understand correcly, this frame would be as stiff as what I'm used to, am I right?
As for frame sizing, compare to Giant, your medium frame for exemple have a much shorter headtube an toptube. Since I'm used to Giant geometry, this is where I have some reserve. How come a 140 mm headtube would be massive vs Giant 150mm? Please highlight me.
The AR4 would be just as stiff as the AR2 but not as light.
I wouldn't characterize 10mm as a "massive" difference. Just ad a 10mm headset spacer, or a shallower drop or a different bend so the hoods are positioned higher or a different stem stack or a flat top handlebar, or a 35mm handlebar or a myriad of other solutions to increasing bar height. It is also possible that you'll find the bar position to your liking. As you are used to Giant geometry, I trust you'll eventually get used to AR geometry.
I had the opposite problem going from a Felt F-series frame to the new AR. I started by using a -17 degree stem but eventually have settled on a -10 degree stem, slightly wider bars, and a slightly lower hood position on the PRO bend. After a few weeks it feels and fits great.
10mm of bar height is about a 2 degree difference in elbow bend.