It is simple. GCC is best c compiler avaible for Amiga.
If 68k is to sloow for code compiled by gcc, 68k should be replaced by faster cpu.
Classic Amiga really need better procesor than 68k.
This procesor should be bigendian beacuse all old software for amiga is bigendian,
PPC is of course ideal choice for classic Amiga, but other like mips, sparc are also welcome.