After testing out epic pet PvP, root landed 100% of the time (about 6x that fight).
On live, the epic pet had
very high MR, at which point it was a useless strategy to try to even root the thing.
Here's a reference on
zam
[You must be logged in to view images. Log in or Register.]
Here is another indirect
reference referring how the pet does in pvp.
Another reference
epic mage pet pvp
I'm still looking for more references, and will need Dr. Nirgon's help on this one.