There is a solution - SIMBL plugin called winBuddy which is available through mySIMBL.
It allows you to turn off window shadow and/or window borders for every app.
I've just tested it and it seems to work properly, there are some minor glitches here and there but nothing major and I think they will get fixed soon.
Minor glitches include:
- You can't change window options for apps which don't have menu - like Bartender for example
- You can't change window options for some apps, as winBuddy does not show up under 'Window' in menu.
I've forked winBuddy and created a version which has window borders set to OFF by default for all apps (that is the look I was going for), maybe someone finds it useful.