Sorry, I forgot something.
Keil recommends to use the same version of the compiler that was used at the development time. New versions of compiler also use new optimization methods (like new: Common tail merging, where common code from different modules is extracted to single instance and in modules replaced with JMP instructions.) Such optimizations in general make changes to the software execution timings (additional JMP's increase the time a bit, better loop optimizations reduce it). It is possible to disable the compiler optimizations or to set it to lower levels.
You will have to play a little with it to find out the right setting or to obtain a original version of the compiler.

Franc