When I dream up a project requiring more grunt than a 16MHz 8 bit ATmega, I know which way I would go. A faster single processor is going to be much much easier to deal with than multiple slower processors. So it would be a Teensy 3.1 or similar, or even janost's Catweazle.
I would only choose multiprocessor for educational purposes. Something like the Teensy should outperform half a dozen ATmegas.
Paul