Re: Arm A53
The Raspberry Pi is more expensive than a core i7 - when measured in performance per £.
Also the Beowulf cluster of Raspberry Pis was a nice hack, but impractical for almost anything since the networking on a RPi is so slow (it goes over the USB interface).
Of course, you could put a bunch of ARM A53 on a die in an advanced technology node, with fast interconnect, and get something which would be like a modern-day transputer. Then you merely need to rewrite your software to be efficiently multi-threadable.