ACAD 20xx is not at all multi-threads / multi-cores.
Today to get the best results with ACAD 20xx (64 bits with 16/24/32 Gb Ram) on large DWGs is to use special PC (very well designed in term of coling) based on the new Intel Core i7 4790K (Basic speed = 4.0 GHz + Turbo +400 Mhz). Cost of the 4790K = around 300-350 $.
And with a god ventirad (Noctua high end for example - You dont ned liquid Water coling), you can easily overclock (without any problem) the Core i7 4790K to a basic speed = 4.5-4.6 GHz + Turbo +400 MHz Its very reliable.
This kind of PC will be faster for ACAD 20xx (and cheaper) than ANY Intel Xeon based workstation.
And the must (for ACAD 201X) in term of graphic card is the Nvidia GTX 780 TI (3 Gb Ram).
Its faster than ANY Nvidia Quadro.