I have theorycrafted a fairly deep understanding of the mercs, and I can still say that without a doubt, a5 barb is far superior. Fire Damage (his Blood Thorns) just can't compete with Barb WDM (Shower of Rocks and Ancients Hand) endgame, and the rest of his skills deal pretty uninteresting damage. They may be comparable in Sigma when Monster Immunities are removed, but i'm still leaning towards Barb merc.
Using TU Trident Sea Summoner and the TU Heavy Gloves are a very effective and cheap combo for constant Glacial Novas, especially if you fill his sockets with PSapphires for cold duration, but his damage will be pathetic.