Moving beyond high end microcontrollers to high performance computation embedded systems
Most microprocessors with gpus end up in embedded computers anyway, but if you want real grunt and aren't afraid of some not-so-user-friendly programming languages, check out Altera's Cyclone V SoC or Xilinx's Zynq, multicore Arm chips coupled with not insubstantial amounts of FPGA logic, DDR3 support and dedicated gpus. For sheer computational grunt (but not on-the-fly flexibility) it's hard to beat an FPGA, you can even compile arduino code for an FPGA. (P.S. you don't have to run a full OS on a pi or a BB, you can always load an RTOS, you can load an RTOS on almost anything with an IO pin, it's just more complicated)