The audio interface is outside the signal path of a DAW's internal mixdown (if not using external gear or loopbacks).
The audio interface will not affect the null test.
As for people hearing differences between v7 and 8, there may be a paradox at play: those with the most golden of ears are most likely to hear, on occasion, differences where there are none.
It takes critical listening and skill to even be working at a level where a mistake could be made.
Another way to put it: the person with the most knowledge and who contributes the most is statistically more likely to say something incorrect than those who don't contribute much, or who play it safe with their contributions.
So there should be no embarrassment for a mastering engineer being wrong about this, this time (if indeed they are).
4 DAW Network:
Main: Studio One V3 (Cubase Pro 9.0.1 on ice until future update solves some issues), Win 7 64-bit, i7-4790K @ 4.6GHz, ASUS Maximus VI Gene Z87 mATX, 16GB, EVGA GTX 760, Focusrite Saffire Pro 40 on a Vantect FireWire 400 PCIe (UGT-FW200), CMC Controllers (2 FDs, PD, QC, CH, AI, TP), 2 NI Kontrol F1 Controllers, Roland JD-Xi, rtpMIDI, Bome MIDI Translator Pro
Node 1 - VSTi Hosting via VEP: VEP 6 (& Sonar Platinum), Win 7, i7-4770K @ 4.0Ghz, Asrock Pro 3 ATX, 16GB, Focusrite Saffire Pro 40, NI Kore 1 Controller, rtpMIDI
Node 2 - 16 Channel Stem Summing via VEP: VEP 6, Win 10, i5-4690K 3.9Ghz, Gigabyte Z97MX, 16GB, Intel HD 4600 Gfx, rtpMIDI
Node 3 - 2 Channel Mastering Chain via ADAT Optical: Reaper, Win 7, i5-4670K @ 4.1GHz, Asrock Pro 3 ATX, 16GB, Nvidia GeForce 210, Focusrite Saffire Pro 40, rtpMIDI