1.5% is the overall Boonton instruments accuracy. That translates to a possible 0.09 dB error @ 6.00 dB amplitude in this discussion. So claiming sub 0.10 dB is not a reliable claim although it is a viable one, but we don't deal in maybes when doing specs, so I can't guarantee sub 0.09dB at this point.
The sensor head is calibrated with 0.01 resolution cal factors. Not 0.10dB.
With regards to repeatable precision, it's fairly stable unless you try to jiggle the connectors/cabling.
I'll address replacing the BNCs as they are likely a source of the impedance matching and VSWR issues I faced. Perhaps just switch to SMAs and leave the BNC/SMA adapter to the user to choose his desired quality.
I can do an exercise on the BNCs and SMAs with my VNA to see the SWR performance. I have a calibrated 2 port (S11,12,21,22) VNA.
I agree about the 5 pin header. I can probably squeeze in a locking type, although I have not found it to be a big issue in actual operation.
-thx for the feedback, Ancel