A couple of thoughts..
One way to answer this is, whoever is going to brew caapi, get a known amount of the brew and extract from it to see alkaloid yield. Then ingest a certain amount of extracted alkaloids one day, and drink the equivalent amount of brew another day, compare the effects, post here.
Another thing to consider: Harmine might turn into harmaline and then into THH over time with boiling. THH does NOT have considerable MAOI activity. People brewing for extended number of hours versus quick extractions might mean that extracted alkaloids are more potent than the brew.
Here's some of the analysis on aya. Seems to vary between 50mg - 350mg of harmalas per dosage (considering a 50-70ml dosage), but often that is repeated 3 times in a session, which would mean from 150mg to 1g+ harmalas in a session (though maybe the strongest ones are not drank so many times or dosages are reduced in subsequent doses).