i decided to parse my pally with and without the thanksgiving pet buff.
i set it up in an instanced Mow's lair and did 2 parses on the practice dummy.
paladin, 4.0 epic, 3 firestrike 4 augs
to make sure that i had the thanksgiving pet buff on me for the full duration of the 10 minute fight i zoned in 6 of my guys and placed them outside the 100 meter range of the pet so they wouldn't even be targeted to get buff.
every 2 or so minutes i'd go down to the pally and cast a pet and walk back up and out of range.
results are in, you can see they are pretty detailed and self explanatory.
for pure melee damage the pet is making no difference at all.
for procs (spells) it's adding 25 damage (50 on a crit).