Do you have any links you could share to some of the studies you're referring to here?
This list is a few years old, ISBN 978-3-9812830-9-9, pp. 119-122. New studies were added at AES149 panels "Time and Perception" and "Goodbye Stereo part II". For this forum, the distinction between "immersion" and "envelopment" might be of particular interest, with ongoing medical studies about to contribute to a better understanding of the latter.
Anyway, additionally to remaining open to new insight, I agree with Kal about objective measurements always being a foundation. A main sentence in the paper reads "objective criteria, not immune to falsification, and repeatable procedures need to be established". Fortunately, that is also what new tools, often developed to understand pathology, can achieve, e.g. certain ERPs, fMRI, NIRS, fNIRS etc.