My impression...
In this scenario it actually looks like more DSP power will address your issues right now (even with your current machine since VI's are not an issue). Native verbs are the power hungry ones. Put those on your card and see how much different things are. Most of the bread and butter stuff (EQ, Delay, Comp in Native) are super CPU friendly (at least the likes of AVID stuff and Softube for instance). Regardless it looks like you are headed to a second HDx card for voice reasons so why not put it into your current set up?
The 12c will of course be a great machine for you and in this application HDx2 + 12 core - why not (its just money
... But after HDx2 even the new hex core might be just fine too.
PS>> in this scenario does buffer 64 really help make the machine more responsive (- really I don't know so no problem if I am wrong but it looks like a lot of extra CPU load for very little gain)? Latency is not an issue. PT11 w/64G ram and fuull timeline cache (is this enough?) looks like more the ticket to super responsive system and kick the buffer to the weeds... 1024