Well, we actually don't. If Shulgin's proposed metabolic action really is happening with elemicin, we should have TMA. TMA has a duration of 6-8 hours, which suites well to what we see with elemicin. The dosage for TMA is said to be from 100 to 250 mg. Ron have, based on his research, suggested 60-70mg as a hallucinogenic dose of elemicin.

Even though each elemecin molecule would have to be paired with an ammonia molecule for the proposed reaction to take place, the additional weight of the ammonia does not account for the large difference in dosages. (225.284 g/mol per TMA and 17.031 g/mol per ammonia = 7.56 % lower dose -> 92 to 231mg elemicin equals 100 to 250mg TMA.)

It might be that there's something wrong in the adviced dosages for either elemicin or TMA (it's an extremely uncommon substance). But if it isn't, then the proposed conversion does not take place. It's always possible that elemicin might be converted to something totally else, but that's a shot in the dark.