* replace Contents/MacOS/Pd with Tk 8.5 Wish.app binary:  /System/Library/Frameworks/Tk.framework/Versions/8.5/Resources/Wish.app/Contents/MacOS/Wish

* add the following to Contents/Info.plist:

<key>NSHighResolutionCapable</key>

<true/>

* change package require Tcl version to 8.5 in Contents/Resources/tcl/pd-gui.tcl

Looking at the app bundle build script, it would only involve updating the wish-shell.tgz with the new Wish.app and renaming all instances of  "Wish Shell" to "Wish" as I found "Wish Shell.app" is now a static link to "Wish.app".

The only problem I noticed is that the font at size 10 or less is so thin that it becomes illegible. I think simply bolding it on retina screens would work.

I wasn't able to find a way in tk to get the screen dpi, so maybe using the max screen size would work:

wm maxsize

If the width and height are greater than, say 2000 & 1500 then bold the small fonts? It's kind of dumb but might work.