Solution
1 - The USRP 2954 SFDR specifications provide only the best case theoretical DAC SFDR of 80 dB. Its daughterboard components will degrade the SFDR and has a theoretical limit of 60 dBc. Unfortunately, NI does not characterize the SFDR of the USRP 2954 transmitter as an integrated device at various Carrier Frequencies, Sample Rates and Tx Power Levels.
2 - NI is planning to remove the DAC SFDR entry in USRP 2954 specification sheet to avoid confusion as it only applies to DAC and not the USRP TX.
3 - Different USRP 2954 at same configuration can have a +-5 dB of SFDR variation. NI does factory adjustments in manufacturing to compensate some IQ mismatches but with time these will be not accurate as the components tend to drift. Other things like tolerances from filter and ICs will also play a role hence there can be differences between SFDRs from different USRPs within the above tolerance limits.