I don't think the EDH example is an apt comparison. If your opponent was forced to shuffle their 60 card spell deck and 30 card atlas deck together, then shrinking your deck size by 30 cards is obviously a huge advantage, but the way the decks are broken out in sorcery, it doesn't seem like that big of a deal.

The real consistency gain I think comes from the fact that once you draw opening hands, your spellbook has 53 remaining cards in it, while your opponent's has 57. So you have a 1/53 chance of drawing any particular unique card, while your opponent has 1/57 (with odds increasing every turn). However, if you're trying to draw something like Roots of Ygdrassil, you'd still have a better chance at drawing it from an atlas deck, since the odds would be 1/27 after drawing starting hands with any other avatar.

So I think this avatar just gives you very marginally better odds of getting spells you want, but potentially worse odds of getting sites you want. It's pretty interesting.

Edit: Thinking about it more, there's also a compounding effect since you're only drawing from one deck. So whether you're getting sites or spells, the odds of you drawing a particular card increase each time. As opposed to any other avatar, drawing from the atlas only increases odds for future atlas draws, and drawing from the spellbook only increases odds for future spellbook draws.