Hi Charlie
I owned a .177 for a few years in Carbine length
Power was just over 10 ft lbs. Accuracy was 1.5 inches at 50 yds off bench(10 shots) with AA pellets
I could have increased the power by adding weight inside piston, but I was offered a (non adjustable)gas ram and so fitted that
Power went up to very close to limit, accuracy stayed the same but cocking effort was excessive. A friends full length one was also modded and was more comfortable to cock
One weakness in the Superstar is the small diameter and hardness of forend screws which can break particularly if a gas ram is fitted
The finish and quality were close to HW77 standard but not quite there and the trigger was very good but then so is the Record trigger
My only other criticism would be the soft laquer finish to the nicely shaped stock is very easily marked(perhaps cellulose)
But, all in all, I think it was a splendid airgun and one of the best British spring rifles ever made. Personally, I may buy another(but full length next time)
ATBest
John