I use an AMD Radeon HD7770 with 3 x 1080p monitors connected, and I don’t have this issue (driver version is Catalyst 15.7). One thing I’ve noticed is that the monitors never go to sleep when CP8 is running, but other than that, things are smooth here. W7 pro x64, i7 2600k + 32GB RAM.
If the GPU related stuff in CP8 is causing the CPU to spike, it maybe that those aren’t coded in the way they are compatible with your card?
According to this thread here :
…there seem to be something going on with the GPU stuff in CP8. The sad thing is, not many people in the thread mention the GPU they are using or the driver version installed. Quickly skimming through the thread, looks to me like there are a couple of people with Nvidia cards reporting editing the registry of Cubase 8 solved the CPU spiking problem. I don’t know if it is the same issue you have, but perhaps worth a look.