Chicago has a Census tract of 508,000 ppsm. It's two buildings. So extremely high densities for Atlanta are possible in this regard. I can promise from living there that there is nowhere that has close to 40,000 ppsm, really not even half. Midtown Census tracts are shaped well to include a concentration of resi high rises, but despite having tracts with 12-20,000 ppsm, the general Midtown area isn't quite as vibrant as similarly dense areas in the standard dense cities. It's the nature of older cities where people live without cars in pretty "crowded" situations (i.e. my 2 BR in SF is 800 SF tops and is considered "luxury" because it has wood floors, lol, whereas a 2 BR in Midtown Atlanta is usually 1,000-1,200 SF and comes with a large garage and amenity deck).
For the SE, Midtown Atlanta is about as dense as it gets, and will only get denser. I see it functioning like a 20,000 ppsm Census tract as experienced in typically dense cities by 2020, which is especially great for Atlanta, but also great because it's very unique in the south (and really the whole Sunbelt stretching to LA). North Ave to 14th will be relatively vibrant and thriving with living options and entertainment options. I lived there as recently as 2012 when Skyhouse (the 1st one Lol) was just finishing up, and I occasionally visit. Right now Atlanta's densest area still has a quiet sparsity to it, and I wouldn't call it "vibrant" despite the density (an equivalent 20,000 ppsm Census tract in SF, Boston, or Philly would be far more bustling). However, I think that's all changing. I see Atlanta's Midtown becoming a little bit like River North in Chicago (over many more decades of course). But Atlanta density DROPS OFF. Even surrounding Midtown there are mansions with yards (Ansley Park, which is one of my all time favorite neighborhoods I should add). Buckhead is literally a vertical suburb surrounded by mansions with the largest yards in the country (outside of CT and western Boston homes).
It's definitely a unique setup. But let's not kid ourselves, despite having Midtown, the city limits density of Atlanta is just over 3,000 ppsm. There's no visible difference between the feel and look of 3,000 ppsm, 3500 ppsm, 2000 ppsm, etc etc. The Atlanta metro is very uniformly "undense" outside of a small core that may have tracts that are "15,000 ppsm +" (but to anyone from SF, Chi, Boston, DC, NYC, LA, Seattle, Philly etc doesn't feel like 15,000 ppsm).
I'll measure Atlanta Midtown density when the skyline when viewed from East-West is "filled in". The day that comes is the day that Atlanta's skyline is *huge* and there are tons of people walking the streets and shopping in stores, etc etc.