It's by CPU only.
Every part of Corona is run by CPU, rendering, interactive, and material preview. I used 4930k workstation to work on that project and mat preview 10+ takes about 20 seconds I think on average.
3dsMax viewport, is run by GPU, but poorly optimized in that fashion. If taken the best iteration of Nitrous (DX11) inside 3dsMax 2014/2015, it still doesn't care much about your GPU. 100 euro 750Ti performs almost same as the most expensive 780Ti for 600 euros. Same goes for Quadro. It's actually quite fast on IntelHD series, only further accenting the fact how poorly it's written. On comparison, Maya Viewport 2.0 (DX11) scales far better on GPU, almost linearly. 3dsMax, not so much. The only reason why 3dsMax appears fast is the semi-inteligent active degradation. Any game engine is 100 times faster...
So if you're building workstation for 3dsMax+Corona, 970 is as high as you should go. Additional GPU power will not give you any benefit at all.