Depends on the circumstances.
If you have a lot of beard/mustache shadow that takes daily shaving plus a lot of makeup to cover it up, it will probably be a BIG help. Beard/mustache shadow is one of the primary things that makes one look more masculine.
If you don't have a lot of beard/mustache shadow (which I didn't,) though, it really won't be a big help passability-wise, but it sure is nice to not have to worry about covering up the shadow with concealer all the time!
In my case, even pre-hormones I had almost no facial hair, to the point that shadow really didn't show up at all unless I let it grow for a couple of days. So on a scale of one to ten it improved my passability by about 2. The only thing it really did for me was eliminate a source of dysphoria and make it possible for me to go out without worrying about makeup. So again, not a passability issue, just a quality-of-life issue. It's really nice to only ever have to tweeze dark hairs or shave maybe once a week at the most.