http://jn.sfn.org/press/April-16-2014-Issue/zns01614005529.p...
http://schizophreniabulletin.oxfordjournals.org/content/earl...
http://www.nature.com/npp/journal/v38/n12/full/npp2013164a.h...
Apart from people getting nauseous or passing out from opiates, I've never heard people complaining about not being able to think. (Sample size ~30). In fact, I've heard the opposite, that opiate treatment improves their ability to think. Whereas with pot, it seems most people are seeking a mental state that doesn't encourage detailed concentration.
Try looking at people's pupils as you go about your day. Doctors, lawyers, executives... I've seen plenty of people with pinned pupils but no obvious signs of cognitive impairment.
I think weed makes people think more abstractly but opiates destroy feeling and complex thoughts.
I can and have written a search engine/database while on opiates. I doubt I could implement cat on pot.
So will agree that it is probably easier for most people to think analytically on low doses of opiates rather than cannabinoids, but I think there are other aspects of mental blocks that manifest strongly on opiates, at least for myself.
There are lots of other downside to opioids, of course, such as dizziness during physical activity and addiction.
Of course, marijuana doesn't really impair me much either. Or alcohol for that matter. My ability to handle intoxication is sometimes distressing.