You can make pets tankier than GDs, or you can make pets have higher DPS than a GD. A leveled Cu Sidhe or Saurosaurus can out tank any GD. Rune Beetles will easily out damage GDs. A Fire Beetle with Rune Corruption and Armor Ignore will smoke any GD/DTH in terms of damage output, especially if someone uses Corpse Skin on the target. Tsuki Wolves are pretty good too, give them Necromancy, Life Leech, and Armor Ignore, and they can dish out some hefty damage, and tank some too due to the Life Leech. They can also spawn with up to 200 DEX/Stam. Only problem with them is that they lower their own melee damage output when they cast Corpse Skin.
Dread Spiders can also out DPS a GD/DTH. I just finished training a Dread Spider, and i'm fairly impressed with their damage output. Got him up to 400 Health/165 Stamina/700 Mana, 300 STR/165 DEX/290 INT, 20 SR/30 MR, 80/80/42/100/63, 24-33 Base Damage, 120 Wrest/GM Tact/120 Resist/120 Anat/GM Poisoning/120 Parry/GM Magery/120 Eval/120 Med/30s Necro (caps at GM)/75 SS (caps at 120)/120 Focus. I also gave him Necromage (after GMing his Poisoning), Armor Ignore and Poison Breath abilities. This is all without the +5k Training Points from the Training Gate bug. His Corpse Skin ups his melee damage, since he does 80% Poison Damage. His Poison Strike has a 9% chance to inflict Greater Poison on any target hit by it, due to his GM Poisoning. His Poison Breath inflicts Lethal Poison on any foes near him due to his GM Poisoning.
Conduit doesn't seem to be working for pets correctly, they cast it, and none of their single target Necro spells will chain in it.
Shadow Wyrms need to be able to reach 120 Wrest and have Corpse Skin removed from them, in order to compete. With just those two changes, they'll out damage GDs, but not be as tanky as GDs (an even trade off i say).