The difficulty with getting a stand in for Peter Cushing would be the cheek bones.
Perhaps a couple of green screen Cheek Pads down to the chin would do it.
I think once you have the cheek bones and narrow gaunt chin you have Tarkin / Peter Cushing obviously with the hair do also. Crooked nose prosthetic and so on.
This model re-creation is also pretty real looking...
http://www.kotaku.com.au/2011/05/you-will-not-believe-these-star-wars-pictures-are-real/
So even if they did do a cgi re-creation they could scan that as a reference so It could possibly be up there with the likes of the schwarzenegger T-800 (terminator 1 model) which was done from scanning a cast that they used to make his fake head where he gouges his eye out.
Personally all CGI is no good though it should be a mix of both real and CGI if they go for a re-creation. especially the eyes are lifeless, so it works for Terminator but not if someone has lines to deliver.