I'm going to say it is neutral or beneficial. That is purely based upon my experience and that of a couple friends. I'm coming off high dose prescribed benzos, and similar the OP I was as high as 70-mg/day. Now down to 20-mg/day by ashton taper.
I'm a mmj patient, I find that whether or not I smoke cannabis it seems to have no effect on my withdrawal symptoms, which are fairly mild. Well, it has one effect - strong indica strains tend to knock me out as if I doubled my benzo dose. So, it does help me with insomnia, but that's not the OPs question. Also, edibles seem to work even better for sleep induction for me if I dose properly and about three hours before bed. I sleep great.
As for brain effects - I don't "feel" any different when I smoke vs. taking a week tolerance break. I'm not more or less clear headed, my memory is about the same. I just don't sleep as well, which could be cannabis withdrawal and / or benzos w/ds.
I've been on benzos for nine years running. I have tapered once before, a decade ago and fully gotten off benzos. Then I got back on nine years ago. 4-mg/day ckonazepam. I smoked the entire time, and for the decade prior to going on benzos.
I feel any brain effects I've had are from the benzos only, that is a very subjective assessment.
There's limited, but hopeful, evidence that cannabis can help heal portions of the brain. GABA specifically, I don't know- not sure anyone has a clue - yet. But I've never heard of cannabis complicating a benzo taper except for those predisposed to cannabis induced anxiety. I've only experienced and heard of good outcomes.
Really - there no solid answer here. Just anecdotal experiences.
I think moderate use is probably ok, but who am I? Just someone who ran out of cannabis today, needs to drive to the dispensary, get more and so I'm reading this forum while I ponder what I'll buy. Probably some edibles for sleep, and indica for evening enjoyment. I only smoke after dark (end of day). Moderate use I think it's best...