What is the range of the input value? If it does not exceed 0-255 then you can place the lookup table in code memory and use the MOVC A,@A+DPTR function to look up the table and retrieve the new value instantly.
If it does exceed 255 then you could have tables for as many times. Then its just a simple matter of selecting which table to use. You'll use quite a bit of code space for the tables but then again it should prove to be very fast opposed to doing hundreds of comparisons.

Hal