It did indeed cornish-stormrider,
Initially, the datum/low switch was designed to be left in 'datum' doing just what it said on the tin, however, over time, it was discovered to extend the engine life considerably if selected to 'low' which, as you say, dropped the max TBT by around 25 deg C. As you say, a combat selection raises it by roughly the same amount, for a slight power boost. I remember that when we were operating a mix of 101 and 103 engined GR1's though, that the 101's were always operated at 'Datum' whereas the 103's would be wirelocked to 'low'.
IIRC, when tweaking the DECU/MECU TBT pot, 25 deg C was also the amount necessary to raise or lower the NH by 1%.
Flipflopman