Hi Jack
The hi-res 5K Mac screens do make everything in Photoshop smaller if not sharper and it's not something that's been addressed (yet) for Macs. My problem is seeing the path on an image, it's a tiny hairsbreadth and the tolerance value around the path points for selection must be only a pixel. Using my Wacom Intros Pro they're really difficult to select.
I've discovered a clumsy workaround using the Mac Finders Zoom preferences feature however. If you go to Accessibility then click on 'Use keyboard shortcuts to zoom' and Zoom Style > Picture in Picture. Click More options and resized the box bigger or leave it as it is.



Now when you're in Photoshop when you activate Toggle Zoom with Command/Opt 8 you'll get a box that follows the cursor around. Command Option +/- makes the zoom level in the box increase or decrease. Unfortunately it's doesn't make tolerances around a point any better it's just a system wide zoom to see things better.
Until Photoshop has a feature to help cursor size and tolerances for clicking around points on hi-res screens, there's not much you can do unfortunately.
Hope this helps a bit.