I think you won't find many because an FPGA that is as fast at computation as a Raspberry Pi will be thousands of pounds. The real advantage is latency and low-level control.
There are cases where an FPGA is used to make a faster CPU. Old CPUs, of course, but it's still a pretty active niche. There are soft cores for Z80s, 6502, and other old CPUs that run circles around the real hardware.