Maybe I didn't explain entirely, consider this the example circuit I was talking about. This comes directly from the old HP e3610 design with a modification not to adjust the voltage divider, but just the reference voltage and the substitution of the LF411 for the OP07.
Assume the measured voltage is 12 V, then if the set voltage is greater than 1.2 V, the output Vout is the saturated output of the op-amp, i.e 12 V. When the set voltage is less than 1.2V, the output of the op-amp saturates negatively, i.e. -12 V.
Vout is connected to their driving circuit which drives the pass element.
The point I'm trying to make is that, in that setup, the op-amp is essentially operating as a comparator. Why not use a much faster comparator instead?