With the +40% productivity, asteroid reprocessing returns more than it consumes.
Normally, you turn 1 chunk into 0.4+0.2+0.2=0.8 chunks on average. With 40% productivity, this turns into 1->1.12 chunks on average, meaning you can turn a couple legendary chunks into infinitely many.

Having 4 slots is already quite powerful, both for normal usage (whether with productivity or with speed/efficiency when you have no space for beacons) and space casinos (if quality was enabled - currently, it's missing from allowed_effects, even in Factorio 2.0), so I'd recommend to remove the bonus - especially since it's not even that relevant for the crushing recipes due to asteroid processing productivity researches.