I think part of the reason it's hard to definitively determine whether there are, in fact, any permanent effects, is because of how difficult it'd be to put together a study like that. We'd need an objective ways to measure cognitive performance both prior to starting benzodiazepine therapy (heck, that alone is difficult since the condition being treated by benzos could cause cognitive issues itself) and after tapering off them, we'd need control groups that metabolize medications the same way, we'd need to be able to definitively determine whether any cognitive issues could actually be signs/symptoms of other illnesses, patients would have to be followed for a lengthy period of time post-cessation, etc. In short, it would be quite the task to even put a study like that together, let alone draw definitive conclusions from. It would definitely be an interesting study, but AFAIK you're right about there being no definitive ones.

/u/ltbenzo, benzos have been around and in use since the '50s. They are very safe medications. For the vast majority of people, any impairments you notice when using them will dissipate after you stop taking them. Also, the amount you mention in your post, 6mg Xanax, while certainly on the higher side is actually what some people are prescribed (prescribing guidelines indicate that 10mg/day is the maximum rx-ed dosage). So, while it IS a higher dose and daily use for 6 months will require tapering to avoid discomfort, it's not as crazy a recreational dose as it may sound (note: I'm not saying you should therefore take 6mg recreationally daily or up the dose, just stating facts).

If you also take benzos therapeutically, the thing I would be most concerned about is increasing your tolerance to a point where you no longer can get therapeutic benefits from lower doses.