To be honest, it depends on what you are computing. Raspberry Pi are made to be versatile, and do a lot of different things. The IoT, personal computers, supercomputers, servers, etc.
If you cluster, you increase the power of your setup with Pi.
there are supercomputers built to hash and process data, built out of pi.
there are way more powerful gpu setups, that will process graphics, and big data as well.
Take for instance, cloud computing, and understand, that you can essentially, create clusters and supercomputers, within cloud framework.
then you should understand, that adding GPUs on google cloud, AWS, AZURE, or Bluemix, increases the price of your running instance.
Many times, it's as expensive, if not way more expensive, just to add a GPU instance.
In google cloud for instance, you can have up to 8 gpu instances, for an 8 core VM instance.
Now, take all the dough you would spend to not only purchase all those raspberry pi, and all the cost of electricity, and understand that you are probably in most circumstances, better off running 1 raspberry pi.
and then just using that raspberry pi, to connect to cloud compute services.
there are demos to try out of cloud computing services, but pretty much, none of them will permit you to try out GPU instances on demo accounts.
SO I would just use a raspberry pi, and run ubuntu mate,
and just connect to IBM bluemix, and or google cloud.
in order to create clusters.
the only thing that bites with that, is that app development in the cloud sucks,
if you need to run xcode, because you can dream on, finding a damn MacOS image,
for the cloud, without purchasing your own, to upload to VMs.
unless you are creating some sort of robotic cluster, that
is motorized, for physical display purposes.
that's my 2 cents.