Usually between 30-40% of his damage is from the pet(s). On Baron Ashbury for example, the ghoul will do around 4k dps over the entire fight. Gargoyle is worth about 3k DPS during it's uptime. Building up the 5 charges of Shadow Infusion as you mentioned can take awhile, the time can vary quite a bit in PvP depending on your time spent CCd/kited or if you opt to use Gargoyle first.

As far as PvP goes, I haven't done much Cataclysm arena yet... still sort of experimenting with various spec combos and trying to figure things out. For what it's worth the few time's I tried Unholy on my 5v5 team I had either died before Dark Transformation would go off, or it would finally pop when it was clean up time against the last 2-3 remaining enemies. I did try boxing some 3v3 as Unholy+Enhance+Holy as well, Unholy seemed perfect for this team, though I think when the patch goes live I'm going to bench the shaman for the Warrior

That said, the Death Rune generation potential for NS or SS is epic and as you said, the tree is getting some neat stuff in the upcoming patch so we will have to see what happens!