That is why I use the 5D3, it takes both CF and SD cards. When using the eyefi card I save RAW to the CF card and the lowest res jpeg to the SD (eyefi) card. With the 5D3 the RAW files can be 27-30MB in size while the jpeg are aroung 750KB. Wireless transfer of the RAW files is therefore much slower with the eyefi card.
With the eyefi card you set up which networks it will attach to. You can setup private networks, Direct mode and what to do with Public Hotspots.
I have it set to work on my home network of course and also on an ad-hoc network. I use a Macbook air and when away or on location I setup the macbook to create an ad-hoc network called "stevesmacbook" or similar and I have already set the eyefi card up to "Attach' to this network when it doesn't find my home network.I think this is using the direct mode.
You also need to specify on the eyefi card where it will save/ send the files to. I have a portable 500GB hard drive that plugs into the Macbook and on that I have a folder called "Today" and this is the folder I specify on the eyefi card. I got caught out a couple of times by creating a new folder on the HDD to save the current photos to and set this folder up as the monitored folder for aperture etc and then couldn't work out why the photos were not showing until I remembered that the eyefi card was sending them to a different folder!
I use Aperture Hot Folder to also monitor that folder and when I run Aperture I set up a new project that uses the same folder.
As I shoot I get a full RAW file saved to the CF card and the S3 jpeg to the SD eyefi card that then automatically sends it wirelessly to the "Today" folder on the HDD connected to the Macbook Air. Aperture Hot Folder sees that a new photo has appeared and shows it in Aperture.The first file when you start up, or after you have had a break for a while, is slow to appear but then the subsequent photos pop up pretty quickly. I guess you would have to test for yourself to see if it is fast enough for your requirements. It probably would be OK for my school shoots but, they are usually pretty hectic so I just didn't want to add the extra "thing" to think about at this stage. When shooting portraits in my "Studio" ( some may call it a double garage) I use the eyefi system all the time - With the macbook air connected to an appleTV showing the photos on a 55inch LCD TV!
Once setup, and it isn't as complicated as it probably sounds, it all works automatically and easily.
As I shoot I watch the photos ( or an assistant does for schools) and picks the "Keepers" by giving them a 1 star rating in Aperture and we also put the persons name in the Title field of the metadata for the choosen photos.
When the shoot is finished I open aperture and look at the jpeg files and filter to only show the "Keepers" I then plug the CF card into a card reader on the mac and import them using the "Matching RAW" option and the "current filter" option. Aperture then imports only the RAW files that match the Keeper photos and the RAW files have the metadata matched. I then select to view the Raw files and away I can go with post and printing.
It all sounds complicated but isn't really. (Thinking of maybe doing a youtube video to show the setting up and use - which would be easier to follow).
BTW - As I said before I have had no trouble shooting tethered into Aperture with the 5D3 before or after Mountain Lion and the issue with Lightroom was that Adobe hadn't updated to include tethered shooting for the 5D3 until they released 4.2. Now they have I can shoot tethered direct to Lightroom. It hasn't been a Mountain Lion issue for me at all.
Hope this helps