EDIT: Actually., this doesn't seem to make much difference to FSX. I must have changed the resolution at the same time.
I just discovered this one-line tweak via the Elite Dangerous forums. Basically it enables supersampling internally in Steam VR:
https://www.reddit.com/r/Vive/comments/4q3aa9/psa_supersampling_on_the_htc_vive_has_arrived/
I just tried it and I'm sure it is now smoother and much better looking than before. I think it means that you can safely disable both types of antialiasing in FI options for maximum performance.