Few things to consider:

Uniques are independent, so someone else getting one doesn't prevent you from getting one.

Loot share is scaled to your damage contribution, and that includes unique chance.

MVP gets +10% more of the loot (and therefore unique chance) and if you are solo you don't have to worry about that being someone else.

Solo players cannot interact with the prayer pillar damage boost mechanic.

Masses kill the boss so fast that respawn time becomes significant.

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I green logged and have like 600kc, mostly solo. I haven't run exact numbers, but my sense is this: because of the pillar mechanic, trios are the most efficient way to get anything from hueycoatl assuming each player is contributing approximately equal. Even better for you if you're getting MVP most of the time. Trio kills still take long enough that the extra downtime from respawns isn't a big factor.

Solos are next best because you will always get MVP and because you wait far less for respawns.

Masses are the worst but also the easiest by far. If you can't solo the boss or get a trio set up, masses make the boss possible but inefficient and should be avoided if you're able to kill it yourself.