As a matter of fact for a home/hobby HPC, or even for development environment, just one standard PC gamer board with GPU would be faster than any set of Pis.
The cool thing here is to set something cheap and didactical I guess, for class, IoT, and practical tests. Moreover there is the question of low power requirement, small size and price. This is a powerful idea, it gonna evolve!
I saw someware that this board to set the 16 Pi Zeros would costs around US$ 100,00...
If a powerful HPC is needed try something such as NVIDIA Tesla or that crazy thing from Facebook:
NVIDIA Tesla
Facebook AI GPU Server from hell!!!
As it runs Linux I think we will be able to run Mathematica and the CUDALink. But it can be price depending on your project/budget.
