Katie,
Tried the Raster Printing option to a shared usb printer (epson cx5200) and a shared, networked, rip softwared (xrip), HPDJ455ca and had the same (thicker) diagonals appear in both cases - after switching to the Raster Printing option with the warning of Very Slow.
Related, may not be helpful, Archicad - granted a few years ago and not OSX - had an option to have either square ended lines or rounded in the Mac environment. They figured it out?
Frank