Holy shock aura stack til lvl 55, so 2 thunderfury with max aura will only have lvl 55 instead of 60.
Nephalem + thunderfury makes you to 45, you can use a dream shield for +10 lvl(but i didnt remember dream didnt add damages).

For the same reason, a barb frenzy wont outperform a zealer pal, but he do have 1 interesting thing: with melee deal splash damages + barb prime, you'll do a lot of AOE damages and you'll stun at the same time, it could be very intersting in crucible or anything else.

You can also make a frenzy barb with TF + frostmourne, obivious reasons, but you should at least wait for 3.2 for the fix on frostmourne effects