Wow, so many good renderers out there at the moment!
I'd say corona's doing more than fine on cpu, it's an incredible engine, not to mention it's ever increasing range of excellent features, and stellar customer service.
My main reason for the request was that looking to the future, gpu's are cheaper to have multiple of, due to many motherboards being able to house up to four, they're cheaper to replace/upgrade as pcie is less likely to change, and even if it does it's generally backward compatible unlike ever changing cpu sockets which often require a new build, and they hold their value well due to a massive gaming market. With all that in mind, if you then consider that even a single consumer gpu can either roughly match a similarly priced cpu in some rendering scenarios and vastly out gun it in other scene types...then it just seems like a good direction to consider.