Hi Capn Bloggs,
if the N1 is spinning at the required speed, then you will have the thrust required,
With RB211 - we had something called EPR shortfall. When trying to set the required EPR, sometimes we couldn't quite make it due reaching N1 or TGT limit first due engine degradation. Hence there was a performance penalty to apply.
Without EPR, you could be happily setting the required N1 without realising you weren't actually achieving the required thrust.