I should start by pointing out that the sum total of my experience of the Superstar was removing the cropped, unfinished aftermarket mainspring from a .25" example yesterday morning, calculating the spring stiffness and preload needed for 11 ft. lb. with the owner's preferred Rhino pellets, shortening the spring and finishing the end, reassembling and testing.
The rifle returned 11 - 11.2 ft. lb., is as consistent as variations in the pellet weight allow, and has a fairly pleasing quick yet gentle shot cycle.
On the basis of my observations, the rifle shoots well in the large calibre, but is let down by the trigger. I would not put it in the same class as an HW77.
Nonetheless, your grandfather's rifle is well worth working on. I would start by checking the piston and breech seals, as in .22" it should not be harsh, suggesting air leakage.