Do companions benefit from wearing +Presence gear?

Solution 1:

No.

Companions do not gain any benefit from wearing gear with +Presence.

To test this, I used two earpieces, both with 28 endurance, and one of which had +6 Presence as well.

When I equipped the +Presence earpiece onto my character, my companion gained +30 to max health. When I switched between the two earpieces on my companion, her health remained constant.

UPDATE: Still no, but for different reasons.

As of recent updates, companion gear is now entirely cosmetic in nature. The only two things affecting their stats are your influence rating with that companion and your Presence stat.