The secret to LB in Dominion is spell vamp (Revolver>WotA), then DFG or Evil Tome before Deathcap even comes to into play.
LeBlanc has some problems very early on in Dom, but due to her extremely high base damage, requiring hard AP is not one of these problems. Rushing Deathcap on LB is a noob trap. I've played her enough on Dom to know it's a very mediocre path. The other items give you more than just AP, and allow you to have some health and mana sustain. The Deathcap rush is a very "kill someone then go back" style for LB that will often not work because you're evening out the situation by having to go back after every engagement.
Land all four of LeBlanc's spells - Sigil, trigger it, Mimic, trigger it, Distortion, Chains - without missing your chain, or wasting distort to get into position, and that's a huge amount of raw damage. This is what gives LB her feared early game.
Once people start buying health, you're going to need AP if you want to keep up, as mages do not scale against max health. People talk about LB's bad late-game, but her scaling is deceptively good, as two of her spells are broken in half. Sigil actually has a .9 AP ratio all together, and Chains is a full 1 AP ratio if both parts trigger (.5 on each part).
The power of DFG is that it adds a fifth spell to your burst rotation. More damage is always good.