Yes, by fitting a smaller pulley you will make the charger unstable at the higher engine rpm ranges...
Therefore the decision was made to keep the 80mm pulley. Also, it is difficult to get more low down power with the standard camshafts, as they don't really "come on" til 4k anyway, whether you've got a charger or not. That being the case, if you wanted more low down power you would need to move to a milder set of cams, but this would compromise the higher rpm power.
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"Supercharged - 454.1bhp/317.5lb/ft"Peugeot 306 GTi-6
2000 (X), Moonstone