who has tried over clocking the at89c51 in KEIL C? how fast can it go? 10% 20% 100%? post your results.
Now, imagine that you are stuck with a 8051 for some reason. What if you wanted to squeeze that extra little bit of performance out of it?
The first step might be to pick a faster, more modern 8051 derivative than that antique the OP mentioned.
Why stick to the outdated 8051 architecture when a much more capable Cortex M0 based chip is as cheap!?