What is the Aperture version that you are using? With my version, Aperture 3.4.3, the completion based on names in "Contacts" still works well.
Do you have the option "Limit suggestions to project" enabled? Then the completion will onl show names that have already been used in the project. If you need to see all contacs, disable the flag.
To export your "Faces" database, the safest option woul be to create an album, that contains a photo of each person that has a named face in your library. Then export this album as an Aperture Library (File > Export > Albumas library") and import this "Faces" library into the library, where you want to use the faces. exporting as library exports the faces entries as well.
Finally, I found the bug why my Contacts database can't work in my Aperture.
My system works under the language of traditional Chinese, and my preferred language orders is:
"Traditional Chinese" -> "English" -> "simple Chinese" previously.
After I exchange the order of "English" and “simple Chinese”, my Contacts works in my Aperture.
Unfortunately, I preferred the Aperture works under the English language.
I had reported it to Apple. Hope that it will fix the problem soon.