The hydraulically operated cowl flaps were more useful in very cold conditions, when they could be closed off to keep CHTs up. One of the big no-nos with those 1830s was letting them get too cold on descent.
In warmer climes hydraulic cowl flaps provided no advantage over the simple fixed cowl opening and is probably why operators modified to the lighter and trouble-free alternative. As far as climb and cruise performance went, the differences were minimal.