Especially when I plan on doing this in C so cannot optimize the assembly code apart from Keil's creation. <p>

That's not quite true. Or at least you can try to avoid many pitfalls in C that would make your program slower than it has to be. Knowledge of '51 assembly is required (and you need to verify the compiler's output).

For example,

while(somechar-- != 0)
{
  DO_STUFF
}

and

do
{
  DO_STUFF
} while(--somechar != 0)

might look very similar in C, the latter is quite a bit faster, especially when DO_STUFF doesn't to a lot.