Copying from Windows works!
I haven't read this whole thread but am going to repost something I posted earlier in the hopes it will save y'all some time:
https://forums.macrumors.com/posts/9887316/
You can avoid having to re-download games you've already installed on a Boot Camp or Windows machine by copying the files over. This worked with Portal so I assume it will with most other Valve games also.
I copied the following files from my Windows 7 install:
portal content.gcf
portal english.gcf
source 2007 shared materials.gcf
source 2007 shared models.gcf
source 2007 shared sounds.gcf
source materials.gcf
source models.gcf
source sounds.gcf
Total size 4.71 GB
This left me with only:
source 2007 mac binaries.gcf (128.6 MB)
that I had to download. So about 3 minutes to copy and another 2 to download and Portal fires up just fine. I even ran the "Verify Integrity of Game Cache" option and it reports everything is 100%.
I figured out which files to copy by starting the Portal install to let Steam create the files on my Mac. I then paused the install and quit Steam, copied over the files from Boot Camp and then restarted Steam.
EDIT:
Files on your Mac are installed under: /Users/
username/Documents/Steam Content/
or in other words: /~/Documents/Steam Content/
Files on your PC
should be installed in: c:/Program Files/Steam/steamapps/
.