Finding the right GPU
I tested almost every GPU from nVidia and AMD and found two viable options. The nVidia Titan V offered the highest capability per watt while the AMD Radeon Pro Duo gave us the best price-to-performance ratio.
The best CPU that would fit
In the mini-itx form factor, the AMD Ryzen CPU gave us the best price-to-performance and performance-per-watt. A Noctua CPU cooler and chassis fan keep the system quiet under load.