Hi Mike,
It is possible, but gContactSync synchronizes the first seven numbers from Google regardless of the actual type (Home, Work, Mobile, Pager, Work Fax, Home Fax, and Other) with the fields in Thunderbird. So, the original WorkPhone field in Thunderbird could contain a mobile number, for example.
I definitely see why you would like the originals if you had a lot of contacts in Thunderbird originally. I will
add a preference (not sure if it will be in the dialog or not yet) to disable this. It's a simple fix, so it should be in 0.2.0b3.
Josh