Would increasing fork length by 20mm and also removing 20mm of headset spacers result in pretty much the same reach?

Fork length won’t change the reach, its the other side of the headset. You will ‘feel’ its a bit more upright though.

Yes it will. Increasing the fork length by 20mm will shorten the reach by just under 10mm. Dropping the stem 20mm lower on the steerer will move the bars about 8mm forwards. Obviously your head and seat angles will end up almost a degree slacker and your bottom bracket about 6mm higher.

What chief groove guru said.

Start with only 10mm less spacers though, as the fork will sag an extra ~7mm or so anyway. e.g 140mm with a ride height of 92mm, becomes 160mm with a ride height of 106mm (not 112mm).

how much reach changes depends on the head angle etc to start with.

Thanks folks, I was after a rough estimate and looks like it is similar, although I take on board the extra sag will mean that ride height will be less than 20mm higher.