I think the reduction to .575" is a bit too much, even for your 'tight' 20 bore. You can or should be able to load a .585" to .590" with no problem with a thick enough patch to hold sufficient lube (.018" to .022"). Your bore is the tightest I've heard of yet.
I was shooting Taylor's Lee .600" mould in my choked .62 that cast .605"x.608" pure lead ball, but ordered my own mould from Jeff Tanner at .595". This size easy loading (.610" muzzle diameter) and there is no need to wipe at any time throughout the day's shooting. In ratio, that would mean .590 to .595" for your gun. It takes a whack with a short starter to start the ball, but goes done easily after that and doesn't foul no matter how many shots are taken. 'm using a .0215" ticking patch.
The larger bores, like 16's through 10's seem to do OK with much smaller balls, ie: .030" or so under bore size, but most 20's I've seen do well or best with slightly larger ratio balls.
Ball size for your bore depends on the lead alloy you are using to some extent. If you want to, or need to shoot very hard balls, then you might have to sue a .575" or .580" ball.
I would try .575's and see what happens. I would not go smaller than that, though, and feel .585" might be about perfect for any hardness lead.