You might want to do a google for learned papers on cutting small holes with plasma - it's a whole subject/artform in itself. That said - 20mm isn't really regarded as small from this point of view.
As Hood has said, if your circles are G2/G3 lookahead isn't an issue. Your hole has a leadin, a circle and a lead out so even if lookahead was an issue it wouldn't matter what you set it to because it's only going to actually lookahead by TWO - the join between the leadin and the circle and the join between the circle and the leadout.
Lookahead like everything else involves a compromise. Make it too small and it's less effective. Make it too large and your needlessly burdening the system. To take Fastest1's analogy. I don't give a toss about how fast I can negotiate the driveway into Hood's mansion if I'm currently driving onto the M5 roundabout - 500 miles away.