Unless you're doing BIG things in Autocad, I suspect you won't notice much difference CPU wise.
I have friends who are using far slower machines on big oil and gas pipeline projects, and most of their speed issues is with the version control system, not the PC they are using.
Just be aware, if you have 8 GB of RAM, OS X will need a couple of that; you won't be able to allocate 8 GB to Windows.
I haven't used Autocad myself for a long time, but I'd be tempted to go for either a Retina or cMBP and get 16GB in there to have a bit more headroom.
If you're planning to be running CAD VMs on this machine during its life, I think only 8 GB of RAM is likely to become a bottleneck within the next 3 years (RAM prices have tumbled - and that means application developers WILL consume it).
If you're only planning to keep the machine for 12 months.... carry on 😀