The power or torque limits for the diff will be based on statistical distribution of MTBF, and conservative so as to minimise failures within a defined lifetime. If you shift what you consider an acceptable failure rate up a little and the mean lifetime down a bit, I'm sure you can stretch to 300ps without too much concern.
i.e. it's more likely your diff will break, but only a tiny bit.
I'm speculating but I wonder if the 10PS difference is because it's a piggy-back ECU and thus modifying the baseline settings by a percentage only, so if you're 10PS down at the start you'll be about 10-12PS down after tuning.