There is no speed improvement possible on rewrititing soft I2C in asssembler, because you must still meet the I2C timing requirements (max 100 kbit).

"There is no speed improvement possible"
correct, however there is a "speed improvement through precision" possible.

1) Who can say what the delay is in a C delay routine without looking at tha assembler.
2) If you have to look at the assembler, why code in C
3) If the compiler changes the code when a newer release is used you may get failures from running too fast or start running slower.
4) Writing the code in assembler an exact cycle count can easily be obtained.

In other words "delay routines should never be written in anything but assembler", I recommend that such routines be conditioned by a defined OSC_FREQ that, if changed, would make the routine disappear, thus creating an error andf alerting the developer to change the routine.

FYI: the above is not assembler fanatism, just good practice.

Erik