I'll bring a little closure to this one. Filed down the brake caliper so I could eliminate the spacer. Looks like it got rid of the the shudder although those blades sure flex when the brakes grab hard. New brakes and new wheels so I'm guessing the extra grab is just flexing those thin-walled blades.

I'm calling this one fixed but will continue to monitor closely. Pics of the surgery:

Disassembled the 5800 brake caliper and filed down the part of the caliper interfering with the headset


Quick touch up with a paint pen


Able to mount with no interference!