What is a modern-day HP-16C?

[Dave Britten]

You're going to have a tough time achieving 'blindingly fast' on a 12 MHz 68000.

I'm not sure if there are any decent C compilers for the 86. I've been told C produces sub-optimal Z80 code, plus the compiler would have to be modified to leave the IY register alone for the most part, due to the way the 86 OS uses it to address the system flags table. It has great support for assembly programs, though, and you can write/install programs that add their own tokens and functions that are usable system-wide.