Okay I ended up ordering a P2715Q to put side by side against my P2415Q because seemingly no one has done this before to compare...?
Bottom line, you can totally tell the difference between the P2415Q's 180ppi and the P2715Q's 160ppi. I'm not sure which one I like better. I definitely appreciate the size of the 27" but the 24" is a solid step in the retina direction that I'll miss when using the 27". At normal desktop viewing distance, the 27 is still amazing compared to a 2560x1440 display.
I also noticed that the color was much more accurate on my P2415, and the P2715 took quite a bit of tweaking to get close enough. P2415Q is also brighter (and warmer) at the same brightness setting than the P2715Q. Other than that, no difference. I got one catastrophically defective A00 model at first, but my replacement is an A03 and my P2715 is an A02. Both have no sleep/wake issues, and both have pretty significant glow at the bottom left/right corners that only shows up on pure black. The panels are rated at 8ms (24") and 9ms (27") response time, but hours of Battlefield 4 on both proved that I couldn't tell the difference.
I think I'm gonna keep the 27" and return the 24", but I've got another few days to decide.