Short answer: afraid to.
Longer answer:

• I had assumed I’d need to use the stock cone-shaped headset cap based on my risk adverse interpretation of owners manuals and supplements.
• My mechanic installed the aero spacers to mimic the use of the stock head cap. This seemed like a good compromise since I was nervous about over-committing with my first ever steer tube cut.

But the pic is already out-of-date:

• While investigating how to resolve my handle bar/ fork rotational issue, I observed that the red rubber seal on the upper headset bearing was visible between the frame and the aero spacer. It didn’t appear either the spacer or the shim that was in there were protecting the bearing from potential ingress of environmental dirt / water.
• I reinstalled the stock headset cone. This part is 5-10 mm taller than the two aero spacers were, and the resulting lessening of slack to the rear brake line has suitably helped with my rotational concern. (The brake edge can still hit the frame, but now it takes purposely turning the bars that far.)

Still to come:

• I’d like to reinstall the trailing aero stem cap but need a new threaded insert. When the prior build was disassembled, this part was removed by screwing it off the frame, leaving behind the thread insert above the top tube, attached through the frame by a bolt accessed from the bottom of the headtube. My LBS didn’t understand what he was looking at or how to get it off, and the painters’ interpretation was that the threaded insert had pulled out of the top tube; so he “repaired it”. Can anyone tell me how to buy a new threaded insert of the right size type? I hope that small part is easier to replace than it would be to order a new aero stem cap part from a supplier in Europe.

Ps: Has anyone ever repaired chips in their painted carbon with nail polish? I’m pretty bummed by the number of blemishes this newly painted frame has already.

---

To breathe, to feel, to know I'm alive.