Almost got a new iMac but went with a new Windows PC. The biggest reason was to give me the ability to deal with things like this. My PC has an NVIDIA GPU. But if NVIDIA can't fix this, I can always replace the GPU with a GPU that works. Apple does't not allow such flexibility.
http://www.dsogaming.com/news/report-total-war-warhammer-runs-27-slower-dx12-nvidias-hardware/
http://www.dsogaming.com/news/report-total-war-warhammer-runs-27-slower-dx12-nvidias-hardware/